Russia tonight warned it would retaliate against Britain if it was hit with a 'punitive' response over the nerve agent attack in Salisbury.
Russia's London embassy launched a Twitter tirade to reject Theresa May's midnight deadline to explain why a Kremlin nerve agent was used in an assassination attempt.
Moscow has demanded to see samples of the Novichok substance found in Sergei Skripal's body before it will consider responding.
Separately, Kremlin spokeswoman Maria Zakharova warned nobody should threaten a nuclear power in an incendiary briefing in Moscow.
Mrs May has been building support among world leaders since announcing last night Britain believed it 'highly likely' Russia was to blame for the attack.
US President Donald Trump tonight told Mrs May in a call he was with Britain 'all the way' and demanded Russia provide 'unambiguous' answers.
Mrs May had already secured the backing of German Chancellor Angela Merkel and French President Emmanuel Macron.
Theresa May(pictured outside No10 today) has been holding talks with world leaders to lay the groundwork for sanctions if the Kremlin fails to come up with an explanation as to how the nerve agent Novichok was used
Theresa May has held talks with French President Emmanuel Macron on the phone last night about the Salisbury attempted murder and will speak with Donald Trump later today
The Prime Minister spoke to German Chancellor Angela Merkel tonight to brief her on the attack in Salisbury
In a series of incendiary tweets tonight, Russia's London embassy said: 'Moscow will not respond to London’s ultimatum until it receives samples of the chemical substance to which the UK investigators are referring.
'Britain must comply with the Chemical Weapons Convention which stipulates joint investigation into the incident, for which Moscow is ready.
'Without that, there can be no sense in any statements from London. The incident appears to be yet another crooked attempt by the UK authorities to discredit Russia.
'Any threat to take “punitive” measures against Russia will meet with a response. The British side should be aware of that.'
Mr Trump told reporters outside the White House: 'Well it sounds to me, I'm speaking to Theresa May today. It sounds to me like it would be Russia based on all of the evidence they have.
'I don't know if they've come to a conclusion, but she's calling me today.'
He added: 'Theresa May is going to be speaking to me today. It sounds to me like they believe it was Russia and I would certainly take that finding as fact.'
Mr Trump added: 'As soon as we get the facts straight and we are going to be speaking with the British today, we're speaking with Theresa May today, and as soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be.
'But I have not spoken to her, I'll speak to her sometime today.'
In a series of furious tweets, Russia's London embassy refused to meet Theresa May's midnight deadline for responding over the nerve agent attack
The tirade claimed Britain was making a 'crooked attempt' to discredit Russia to cover up what really happened in the attack
The Twitter account warned darkly that any punitive measures against Russia would be met with a response
Following the call with Mrs Merkel, Downing Street said: 'They discussed the pattern of aggressive Russian behaviour and agreed it would be important to act in unison with allies to counter it.
'Chancellor Merkel condemned the attack and said she stood in full solidarity with the UK.
What is the Novichok nerve agent used against the Skripals?
The Novichok nerve agent used against former Russian spy Sergei Skripal and his daughter Yulia is among the most deadly poisons ever created.
They were secretly developed by the Soviet Union during the height of the Cold war in the 1970s and 1980s.
Communist scientists developed the poison so it would not be able to be detected by Nato's chemical detection equipment.
They come in the form of a ultra-fine powder, Novichok is up to eight times more potent than the deadly VX gas.
Victims who are poisoned by the powder suffer muscle spasms, breathing problems and then cardiac arrest.
There is a known antidote to the nerve agent - atropine can block the poison.
But doctors find it very tricky to administer the antidote because the dose would have to be so high it could prove fatal for the person.
Novichok poisons are highly dangerous to handle, requiring the expertise of skilled scientists in a sophisticated lab.
Dr Vil Mirzayanov, former Chief of the Foreign Technical Counterintelligence Department at Russia’s premiere, was among the team of scientists who helped develop the agent.
In an article about the lethal weapon, he wrote: 'They are extremely dangerous – most likely lethal – for people who would try to synthesise or manipulate them without the help of highly experienced scientists and engineers in special laboratory installations observing extreme safety measures.
'Without exception, Novichok weapons cannot be used for any reason without specially trained military personnel under medical supervision.'
'They agreed that the international community should coordinate closely as the investigation developed and in the wake of Russia's response.'
Senior MEP Guy Verhofstadt offered EU support and said: 'We stand shoulder to shoulder with the British people.
''It must be made clear that an attack against one EU & NATO country is an attack on all of us.'
It comes as the clock is ticking on the midnight deadline Mrs May has given for the Russians to explain how their nerve agent was used in an attempted murder on the sleepy streets of Salisbury.
British officials are drawing up a plan to hit back hard at Russia if - as expected - they do not have an explanation.
The Government is expected to draw up a list of tough sanctions that will target the Kremlin and Putin's cronies, freezing assets and stopping them from travelling to the UK .
They are also expected to expel many of Russia's diplomats - although probably not break off diplomatic relations altogether - with the country.
British officials and the Royal family are expected to boycott the football world cup in Russia later this year.
But Britain will need to get support from the country's UN and Nato allies in order to impose sanctions that will really bite and be felt by the Kremlin.
The PM yesterday dramatically pointed the finger at Putin for the poison attack saying it was'highly likely' it was ordered by the country.
The Prime Minister said the facts increasingly suggested Russia was behind the apparent 'hit' on double-agent Sergei Skripal in Salisbury.
Branding the attack a 'reckless and despicable act', Mrs May said the substance used was a 'military grade' agent Moscow has produced.
Mrs May said the government would not accept such an attempt to 'murder innocent civilians on our soil'.
Foreign Secretary Boris Johnson said allies are mobilising to support the UK and hit back at Russia.
Speaking today he said: 'I've been very encouraged so far by the strength of the support that we are getting.
Home Secretary Amber Rudd (pictured left) has chaired a Cobra meeting this morning while Boris Johnson (pictured right) said he has been talking to allies to talk about Britain's response to Russia
Sergei Skripal (left) and his daughter Yulia (right) have been in a critical condition since they were found unconscious on a bench outside a shopping centre in Salisbury on March 4
'I think in particular from President Macron of France, I talked to Sigmar Gabriel my German counterpart, and from Washington where Rex Tillerson last night made it absolutely clear that he sees this as part of a pattern of disruptive behaviour, increasingly disruptive behaviour, malign behaviour by Russia, the reckless use of chemical weapons, the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury.
'I've been encouraged by the willingness of our friends to show support and solidarity.'
Home Secretary Amber Rudd oversaw an emergency Cobra committee into the crisis this morning.
And senior MEP Guy Verhofstadt said: 'We stand shoulder to shoulder with the British people.'
She said: 'The Russians have started responding. The Prime Minister has been very clear that they have until midnight tonight to satisfy her requests.
'Until then we will wait and see what they have put forward.
'I know that international allies have begun to rally their support and make comments publicly but at the moment what we are doing is awaiting the Russian response before stepping up and responding as the Prime Minister has said we will.'
Whitehall sources said yesterday they were accelerating their offensive cyber programme and could hit select targets for a specific effect.
Police have put a forensics tent over the parking meter outside Salisbury's Sainsbury's store amid fears it was used by Sergei Skripal
Vladimir Putin said in a Russia TV interview he could forgive nearly everything, but not 'betrayal' - but the Kremlin has denied plotting to kill Sergei Skripal
It is understood this could see a specialist cyber unit deployed in the UK to attack Kremlin computer networks spewing Russian propaganda and trolling factories spreading fake news.
Together with Russia's previous actions and tactics, including the killing of Alexander Litvinenko, the UK authorities yesterday concluded it was 'highly likely' to be involved in the episode.
What action could Britain take against Russia if the Kremlin is shown to be behind the poison spy plot?
Britain could expel Russia's ambassador and other diplomats based at the embassy in Kensington Palace Gardens - round the corner from where Prince William and Kate live.
If MI5 have tabs on a Russian spy ring in the UK, Britain could take this poison plot as a reason to expel them.
Britain already has an extensive range of sanctions against Russia as a result of the invasion of Crimea and eastern Ukraine, but we could extend them.
The UK could also impose sanctions on named individuals if they are linked to the murder attempt.
Britain could pass the 'Magnitsky List' mirroring US laws imposing travel bans on senior Kremlin officials responsible for the death of Russian accountant Sergei Magnitsky in a prison in 2009.
Britain could enact the Magnitsky amendment, imposed conditions on Putin's cronies. The act is named after Sergei Magnitsky, a lawyer who died in custody after exposing corruption
World Cup boycott
Official representation could be withdrawn from the World Cup in Russia if Kremlin links are proven.
Prime Minister Theresa May told MPs last week that the Government would “look at whether ministers and other dignitaries should attend” the tournament in that event.
The Duke of Cambridge has already said he has no plans to attend.
There have been calls for the England team to boycott the tournament but their withdrawal would probably have little impact.
A coordinated protest, involving the withdrawal of multiple countries, would be far more effective in damaging Vladimir Putin’s showcase international event. However, experts say the nerve agent poisoning is unlikely to create enough international momentum to trigger a wider boycott.
Statement of condemnation
Britain may call on the support its closest allies if there is evidence of a Russian murder attempt on UK soil.
A joint statement of international condemnation could be issued from leaders including French President Emmanuel Macron and Germany’s Angela Merkel, warning Russia that such actions will not be tolerated.
In a tough statement updating MPs after a meeting of the National Security Council, Mrs May raised the prospect of significant retaliation - making clear that the UK is already consulting Nato and other allies.
'It is now clear that Mr Skripal and his daughter were poisoned with military grade nerve agent of a type developed by Russia,' Mrs May said. 'This is part of a group of nerve agents known as novichok.'
Another option is for the Government to implement a British version of the US's Magnitsky Act, which lists Russians involved in corruption and human rights abuses, banning them from entering the country.
There could be fresh sanctions against senior figures in the Putin regime, either with the EU or unilaterally.
General Sir Chris Deverell, commander of Joint Forces Command, has revealed how the UK has a specialist unit which is dedicated to 'offensive cyber' run jointly by the MoD and GCHQ.
So far it has worked on Islamic State but this could be expanded towards Russia.
In an interview with the Mail last week, he said the military could hit back at disinformation spread by Russian trolling factories.
He said: 'There are two ways you could respond. One is putting your own messages out to compete with the messages that actors like that are sending. And the other is with a cyber-attack.
'Whether or not you could use cyber as a weapon would depend upon the specific circumstances and the law.' He said that there was a specific capability in which troops tackle mistruths spread by enemies.
Russia's ambassador was summoned to the Foreign Office at 3.45pm yesterday for a 'cool but firm' meeting with Boris Johnson. There was no handshake between the politicians as Mr Johnson outlined the 'outrage' felt by the British people.
She added: 'Based on the positive identification of this chemical agent by world leading experts at the laboratory at Port Down, our knowledge that Russia has previously produced this agent and would still be cap able of doing so, Russia's record of conducting state sponsored assassinations, and our assessment that Russia views defectors as a legitimate target for assassination the government has concluded that it is highly likely Russia was responsible for the act against Sergei and Yulia Skripal.'
Mrs May said Boris Johnson had summoned the Russian ambassador in London yesterday afternoon and informed him of the findings.
The Kremlin was given a deadline of midnight to respond to the evidence and the government could outline its 'detailed' retaliation later today.
'Should there be no credible response we will conclude that his action amounts to an unlawful use of force by the Russian state against the United Kingdom, and I will come back to this House and set out the full range of measures that we will take in response,' she said.
'This attempted murder using a weapons-grade nerve agent on a British town was not just a crime against the Skripals, it was an indiscriminate ad reckless act against the United Kingdom, putting the lives of innocent civilians at risk,' she said.
'And we will not tolerate such a brazen attempt to murder innocent civilians on our soil.'
Russia's Foreign Ministry posted this sarcastic response on Twitter this morning
But the Russian Foreign Ministry showed little sign of readiness to explain itself, immediately deriding Mrs May's remarks as a 'circus show'.
The ministry appeared to reference Mrs May's address to MPs with a video of a snow covered traffic jam, accompanied by the hashtag Highly Likely Russia, attempting to make light of the poisoning.
A clip of vehicles struggling in the snow as the Beast from the East was posted by the government branch, suggesting the extreme weather was Russia's fault.
And the video finishes with a penguin playing in snow, with the caption, 'At least penguin enjoys it.'
Sergei Skripal and his daughter Yulia (pictured in Zizzi in 2016) left a trail of nerve agent in the restaurant after their poisoning
Locals have said chemical weapons experts are removing a potentially contaminated vehicle from a local business
US Secretary of State Rex Tillerson backed Britain - but hours later it emerged that Mr Trump has sacked him.
Mr Tillerson said: 'We have full confidence in the UK's investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week.
'There is never a justification for this type of attack - the attempted murder of a private citizen on the soil of a sovereign nation - and we are outraged that Russia appears to have again engaged in such behaviour.
'From Ukraine to Syria - and now the UK - Russia continues to be an irresponsible force of instability in the world, acting with open disregard for the sovereignty of other states and the life of their citizens.
'We agree that those responsible - both those who committed the crime and those who ordered it - must face appropriately serious consequences.
'We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses.'
Counter-terrorism police and intelligence officers are thought to have presented compelling evidence at the NSC meeting that Moscow ordered the hit in Salisbury over a week ago.
The nerve agent - Novichok - that contaminated the victims was so secret that it was never known to have been used until now.
The investigation into the poisoning has led to a series of locations around Salisbury being sealed off and decontaminated
Soldiers in Hazmat suits closed down a village near Salisbury yesterday as they removed a recovery truck thought to have towed Mr Skripal's car from the scene
Anti-terror police also took over a Sainsbury's car park in Salisbury as part of the rapidly expanding chemical attack probe
Theresa May has offered Russia a way to 'extricate themselves' from the Salisbury attack fallout by saying poison could have found its way into rogue hands, expert says
Theresa May talks in the Commons yesterday
Theresa May has offered Russia a way to 'extricate themselves' from the Salisbury attack fallout, an expert has said.
The Prime Minister announced on Monday that a military-grade nerve agent Novichok was used in the attempted murder of Sergei Skripal and his daughter Yulia.
She told MPs that there were only two possible explanations - either Moscow was behind the attack or it had lost control of its stockpile of the poison.
Dr Jonathan Eyal, associate director at Royal United Services Institute (RUSI), said that by offering the second option she was giving Russia an opportunity.
'It was a two-pronged message - it was a message suggesting that the British government is not prepared to pretend any longer that we do not know who is the culprit in this story,' he added.
'It was also an attempt to suggest to the Russians that they still have a chance to extricate themselves from the fallout.
'It was a rather clever attempt to delay what is likely to be a very difficult period in the relationship between the two countries.'
He continued: 'It's an interesting approach, she could have stood up in the House of Commons and said today that she was advised that it came from the Russians, a special chemical agent, and here is the British retaliation - she didn't do that precisely because it is clear that the downturn in the relationship is going to be pretty severe.'
The Kremlin has denied the involvement of the Russian government in the nerve agent attack on the Skripals.
Dr Eyal said the type of chemical agent used 'would not have been produced by a rogue person'.
'But the Prime Minister gave the possibility to the Russians of suggesting that there might have been rogue elements within the intelligence community,' he added.
Mrs May was giving the Kremlin a 'more elegant way out should it wish to take it', he said.
'Sadly I don't see any chance of the Russians taking it.
'The outcome will be that by Wednesday I think the Prime Minister will be in a stronger position to claim that all options have been explored and there is no escape from retaliatory measures.'
Former Tory leader Iain Duncan Smith branded Russia a 'rogue state' and demanded the 'most severe' response.
He said: 'If we appease a country like this, then we should expect even worse.'
Commons Foreign Affairs Committee chair Tom Tugendhat said the use of nerve gas was a 'war like act'.
Home Affairs Committee chair Yvette Cooper said a cross-party stand was needed against Russian aggression.
As Mrs May delivered her statement in Parliament, the Army was working to clear up Salisbury
In an interview with the Evening Standard earlier, Home Secretary Amber Rudd pointed out that Mr Putin had jibed in a Russian TV interview about not being able to forgive 'betrayal'.
She said: 'I'm not going to enter into a kind of great big tit-for-tat with them, which is what they are longing I'm sure for us to do.
'Because when attribution comes we have to be absolutely cool-headed about it. Other people can carry on making their comments.
'I think that the general public are wise enough to take a dim view of that kind of childish joshing.'
Mrs May hinted that the prospect of pulling the England team out of the football World Cup was not being considered.
Asked about the team, she suggested officials could boycott the event but did not go further.
The dramatic moment in the House of Commons came after Public Health England stepped up their response by warning members of the public in Salisbury to wash their clothing and possessions – seven days after Mr Skripal and his daughter Yulia fell ill.
Three Cabinet ministers - including Boris Johnson - are understood to have been be privately pushing the PM to respond with 'tough action' after claims Britain's response to the 2006 murder of Russian ex-spy Alexander Litvinenko had not been enough.
Ministers, spy chiefs, police and the head of the military were at the talks today on the progress made in the investigation.
Military scientists at the Porton Down research unit expect to be able to say beyond doubt that the rare nerve toxin was devised in a laboratory in Moscow.
The PM is thought to have canvassed views from Foreign Office officials and Air Chief Marshal Sir Stuart Peach on the appropriate response if the evidence proves conclusive.
Possibilities include 'full spectrum' retaliation across diplomatic, economic and military fronts such as the immediate expulsion of senior Russian diplomats and spies, potentially cancelling Kremlin-linked oligarchs' visas, and asset freezes and travel bans.
The Government is also expected to meet MPs 'half way' on implementing a British version of the US's 'Magnitsky Act' which lists Russians involved in corruption and human rights abuses who are banned from entering America.
Senior Whitehall sources said the Government could also plan long-term consequences such as boosting Britain's military deployments in eastern Europe.
Soldiers in Hazmat suits on Saturday loaded up and removed an ambulance believed to have taken Miss Skripal to hospital
Further options include covert consequences for Moscow that will never be revealed such as an offensive cyber-attack.
Marina Litvinenko, whose husband Alexander died in London after being poisoned, said the Government had promised her such a crime would not be repeated.
'Unfortunately, it happened again. It means something was not done,' she said.
The PM's spokesman said senior British officials have been speaking with top officials from Britain's Nato allies and further talks are expected in the coming days.
Asked specifically if she was using 'Nato or UN' language in her statement, he added: 'That is not an Article 5 type statement.'
He also signalled that the Government could back amendments being tabled by MPs to a Bill going through Parliament to beef up the power to impose sanctions on cronies of Putin using dirty money to love the high life in London.
Air Chief Marshall Sir Stuart Peach (left) and Defence Secretary Gavin Williamson arrived together for the NCS meeting today
Home Secretary Amber Rudd and Foreign Secretary Boris Johnson were both at the National Security Council meeting in Downing Street today
Timeline of Sergei Skripal's poisoning
Sunday, March 4th - 4.15pm: Wiltshire Police find a man and woman unconscious on a bench at the The Maltings shopping centre in Salisbury and cordon off the area
Monday, 5th - 11am: Salisbury District Hospital, where the pair were taken, declares a major incident and its A&E department is closed.
8pm: Police officers are first seen outside Mr Skripal's home in Salisbury
10pm: Police close a Zizzi restaurant near the shopping centre.
Tuesday, 6th - 11.30am: Police also cordon off the Bishop's Mill pub in Salisbury, where Mr Skripal and his daughter may have gone after leaving Zizzi.
9pm: Firefighters in Hazmat suits are sent to an ambulance base in Amesbury, eight miles away from the scene where they were found.
Wednesday, 7th - 3:30pm: Cordon around Mr Skripal's house is extended to the top of the cul-de-sac.
Thursday, 8th - 2pm: Police were revealed to have cordoned off the graves of Mr Skripal's wife and son in Salisbury.
2pm: Police also extend the cordon around Mr Skripal's home from 50 yards to 150 yards and around the corner.
7.30pm: Police in protective gear go to Ashley Wood Recovery in Salisbury to examine a maroon BMW-3 series, the same car driven by the former spy.
Friday, 9th -10am: Military convoy of 180 troops arrives in Salisbury, including chemical weapons experts, to join the investigation.
3pm: Detectives in Hazmat suits descend on Salisbury cemetery and removed items from Mrs Skripal and her son's grave.
Sunday, 11th - The army remove police cars and ambulances thought to have been contaminated.
Monday, 12th - Army close off village of Winterslow and Sainsbury's car park in Salisbury to remove vehicles.
A group of backbench MPs want the Government to bring in a Magnitsky Law, which has already been introduced in other countries including the US.
The PM's spokesman said the Government has already passed legislation to beef up the sanctions regime.
But he added that Mrs May 'is looking to build the broadest possible consensus and conversations will take place with those who have tabled the amendment'.
Nato Secretary General Jens Stoltenberg said the organisation was concerned by the incident and that it was backing Britain.
He said: 'The United Kingdom has concluded that Sergei Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia.
'And Prime Minister Theresa May stated today that it is highly likely that Russia was responsible for the act. The use of any nerve agent is horrendous and completely unacceptable.
'The UK is a highly valued ally, and this incident is of great concern to Nato. Nato is in touch with the UK authorities on this issue.'
While Russia is not a Nato member, France is, and Theresa May spoke to her counterpart President Emmanuel Macron yesterday to keep him in the loop.
A Downing Street spokesman said: 'The Prime Minister spoke to President Macron of France to update him on the latest situation regarding the incident in Salisbury on 4 March.
'She outlined the conclusion reached by the Government that it was highly likely that Russia was responsible for the act against Sergei and Yulia Skripal.
'They discussed the wide pattern of aggressive Russian behaviour and agreed that it would be important to continue to act in concert with allies to address it.
'President Macron condemned the attack and offered his solidarity with the UK.
'They agreed that the French and British governments should co-ordinate closely as the investigation developed and following Russia's response.'
'Paralysis, convulsions, you can't breathe and after that you die': Scientist who helped create the lethal nerve agent Novichok used to attack Russian spy in Salisbury reveals its terrifying effects
The Russian whistleblower who exposed the country's secret chemical weapons programme has revealed the horrific effect of the Novichok nerve agents on their victims.
Vil Mirzayanov described the use of the lethal toxins as a 'brazen' attack by Vladimir Putin, who 'thinks he can use everything to kill enemies'.
Mr Mirzayanov says a large dose of Novichok 'paralyses' victims before 'it causes convulsions, you can't breathe and after that you die'.
Vil Mirzayanov (pictured) described the use of the lethal toxins as a 'brazen' attack by Vladimir Putin, who 'thinks he can use everything to kill enemies'
The exiled scientist shocked the world in 1992 when he revealed that promises by the Soviet Union to reduce its chemical weapon stockpiles were hollow.
He worked in the top-secret Moscow laboratory where a new generation of even more potent poisons was being perfected.
These gruesome chemical weapons, named 'Novichok' after the Russian for 'newcomer', were designed to be even more lethal than VX or sarin.
At the time, one former top Soviet military adviser described them as 'political weapons', adding: 'They have a powerful moral and psychological effect.'
Shockingly, they can be created from common, unrestricted and undetectable industrial and agricultural chemicals available worldwide.
Vil Mirzayanov described the use of the lethal toxins as a 'brazen' attack by Vladimir Putin (pictured), who 'thinks he can use everything to kill enemies'
As a result, weapons inspectors fear other rogue nations, including Syria and North Korea, could have their own lethal stockpiles of the powerful nerve agents.
Speaking from his home in New Jersey last night, Mr Mirzayanov, 83, described the top-secret laboratory as a 'criminal enterprise'.
'It's a brazen attack,' he said. 'Putin thinks he can use everything to kill enemies. They don't tolerate any opponents.
'They should be punished. It's an open demonstration of this Russian terrorism.
'The Russian government is telling people who are thinking about revealing more secrets that they can expect the same fate.'
Asked how the nerve agent works, he added: 'It's for paralysing people, it causes you convulsions and you can't breathe and after that you die. If you get enough of a dose of it.
'It's real torture, it's impossible to imagine. Even in low doses the pain can go on for weeks. You cannot imagine the horror, it's so bad.'
The Novichok family of nerve agents were secretly developed over two decades at a research facility 50 miles outside the Russian capital.
Many times more potent than other better known chemical weapons, Novichok agents can render gas masks and protective equipment useless.
One of the few labs capable of producing a nerve agent like that used in the attack on a Russian double agent in the UK is in Russia's foreign intelligence headquarters, it was claimed today
Sometimes described as 'gases' they are in fact liquid, intended to be delivered as a fine spray.
A series of poisons, known as Novichok 5, 7, 8 and 9 to identify them, were produced amid conditions of complete secrecy.
They all kill the same way. By inhibiting enzymes that control nerve receptors in the brain.
One expert said victims simply 'forget to breathe'. A tiny drop, almost undetectable, placed on the skin or inhaled can cause death within minutes.
Describing his work, Mr Mirzayanov said: 'They were normal laboratories, they were not underground or anything. They were testing and developing.
'There were around 1,000 people working on this, it was a big deal. You have to test it on animals and after that you have to study the chemical properties... so many laboratories were involved.'
In 1987, one physicist at the laboratory was saved despite being exposed to the chemical when a ventilator stopped working. Witnesses described how he staggered out of the room, describing seeing bright hallucinations before collapsing and being rushed to hospital by the KGB. He was left with permanent injuries after being critically ill for ten days and unable to walk for six months.
Last night experts described nerve agents such as Novichok as second only to the 'atom bomb' as the most deadly weapons ever made. They said that because the ingredients were so common, the poison was ideal for use in an assassination, as well as a weapon of mass destruction.
Pharmacology expert Professor Gary Stephens said: 'This is a more dangerous and sophisticated agent than sarin or VX and is harder to identify. It causes a slowing of the heart and restriction of the airways, leading to death by asphyxiation.
'One of the main reasons these agents are developed is because their component parts are not on the banned list. It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier.'
Professor Robert Stockman, of the University of Nottingham, said traces of nerve agents did not linger. He added: 'These agents react with water to degrade, including moisture in the air, and so in the UK they would have a very limited lifetime. This is presumably why the street in Salisbury was being hosed down as a precaution – it would effectively destroy the agent.'
Military personnel in chemical suits searching the car park behind Salisbury Police Station
Russia went into denial early today, with a former FSB chief claiming scientists at Porton Down had poisoned ex-spy Sergei Skripal and his daughter Yulia.
A top Moscow chemical weapons expert echoed the charge while also alleging nerve agent novichok did not exist.
Moscow was scrambling today to respond to Theresa May's demand for an explanation for the Salisbury attack by midnight.
In an extraordinary blast, Vladimir Putin's predecessor as FSB secret service chief Nikolay Kovalyov claimed Porton Down government laboratory - which identified Russian-made novichok as the agent that poisoned the Skripals - was likely to have been behind the attack.
'There is a laboratory (near Salisbury), which is famous all over the world,' he dismissively told Britain.
'Check if anything leaked from there.'
Soldiers in Hazmat suits yesterday loaded up and removed an ambulance believed to have taken Miss Skripal to hospital
Meanwhile, chemical weapons scientist Dr Anton Utkin - a former UN inspector in Iraq - denied the existence of novichok, and echoed the claim that Britain maybe responsible for the poisoning.
He led the destruction of chemical weapons in Russia.
'I was dealing with elimination of Russian chemical weapons and with all the responsibility I can say that we have never had a weapon under this name,' he said.
Actually, it is very strange that British experts have found out the formula of this 'super secret' poison nobody knows about. It is not clear either how they managed to determine that it was produced in Russia.
'If you know the technology, it is not possible to find out who made a chemical weapon – if the method is the same, the formulas of the substances will be identical. So this news only provokes more questions.'
Utkin said the West had earlier made a 'toxic noise' about Russia's alleged use of chemical weapons in Syria.
'And now right in the heart of Great Britain this traitor is killed with a neuroparalytic substance.
'All this happens at a very convenient time – before the Russian presidential election and World Cup. If I were British, I would not blame Russians but check my own security systems first.
'Something strange is happening there – it is not the first time when criminals are walking around the country with highly toxic and radioactive substances and the secret services have no idea about it.'
A police car was loaded up to be removed yesterday as the Army and emergency services continue the clean-up operation
Dr Utkin has worked for the State Scientific Research Institute of Organic Chemistry and Technology which is suspected of being behind the development of novichik starting in