EU leaders today promised the Irish backstop plan will be temporary as they unveiled their last-ditch plan to help get Theresa May's Brexit deal over the line tomorrow.
But Donald Tusk and Jean-Claude Juncker refused to change the text of the Brexit deal - meaning their promise does not have legal force.
And it was immediately panned by Brexiteers and the DUP who pointed out it is not legally binding, while Remainers also dismissed it as a 'fig leaf'.
The letter, penned by the Presidents of the EU Council and Commission, was part of a last-ditch scramble to try to peel off Tory rebels ahead of tomorrow's crunch vote.
Number Ten fear the PM could suffer the biggest Commons defeat ever when her Brexit plan is voted on by MPs tomorrow night.
As part of an orchestrated bid to drum up desperately-needed support, No10 published the letter to reassure rebels the hated backstop will not be permanent.
The letter states: 'As you know, we are not in a position to agree to anything that changes or is inconsistent with the Withdrawal Agreement, but against this background, and in order to facilitate the next steps of the process, we are happy to confirm, on behalf of the two EU Institutions we represent, our understanding of the following points within our respective fields of responsibility.'
Donald Tusk (pictured left) and Jean-Claude Juncker(pictured right) have published a formal letter designed to give last-ditch assurances which they hope will persuade Tory rebels to back the PM's Brexit deal
They add: 'The European Council also said that, if the backstop were nevertheless to be triggered, it would only apply temporarily, unless and until it is superseded by a subsequent agreement that ensures that a hard border is avoided.'
They also promised to start negotiating a trade deal immediately as they again promised 'the backstop would only be in place for as long as strictly necessary'.
And they pledged to look at using the latest technology to try to keep the Irish border soft.
But the letter sparked scorn from Tory Brexiteers, the DUP and Remainers campaigning for another referendum.
May warns MPs voting her Brexit deal down would be the height of recklessness
Theresa May launched a desperate last-ditch bid save her Brexit deal today - warning that voting it down would be the 'height of recklessness' and might mean staying in the EU.
The Prime Minister pleaded with Tory Eurosceptics to look at the mounting revolt by Remainers, and realise that Parliament is ready to block the country from crashing out.
In a speech just 36 hours before the titanic Commons showdown, Mrs May said it was clear some politicians would use 'every device' to stop Brexit happening.
Underlining the dangers of the crisis wracking Westminster, she urged MPs to consider the 'consequences' of their actions for people's faith in democracy.
Mrs May also pointed to a letter from EU commission chief Jean-Claude Juncker and council president Donald Tusk published today, which insists the Irish border backstop - the most controversial part of the Brexit deal - will only be 'temporary'.
'We have secured valuable new clarifications and assurances,' Mrs May said, while admitting that the commitment 'did not go as far' as some MPs wanted.
The desperate entreaty came as tensions escalate in Westminster ahead of the vote on the Brexit package she has thrashed out with Brussels.
Mrs May rejected the idea that alternatives to her Withdrawal Agreement were available.
'Nobody has yet come up with an alternative Brexit deal that is negotiable and that delivers on the result of the referendum,' she said.
'The only deal on the table is the one MPs will vote on tomorrow night.
Nigel Dodds, DUP deputy leader, told the BBC: 'The letter isn't legally binding. The Prime Minister will struggle to justify what the delay was about.'
Tory MP Steve Baker, shop steward of the Tory Brexiteers, said: 'These new letters do not change the legally binding surrender that is the Withdrawal Agreement.
'A letter or codicil from the EU Commission and Council cannot replace the commitment in international law that the draft Withdrawal Agreement represents.
'Without changes to the text of the draft agreement, the United Kingdom can still be trapped in the backstop for decades without any means of escape.
'The letter cannot change the status of the draft Withdrawal Agreement from that of a legally binding international treaty obligation.
'Without changing the text of the draft agreement, passing the Withdrawal Agreement would still leave the UK trapped under the EU's thumb in the backstop indefinitely with no way to leave.'
He said there are also major concerns including the £39billion divorce bill.
Tory MP and leading Remainer said he does not think the letters change anything, and that he still thinks 'this is a third rate future for our country if we leave on these terms'.
Ian Murray, Labour MP and a campaigner with the anti-Brexit group Best for Britain, said: 'This letter is nothing but a fig leaf that fails to cover up the massive shortcomings in the Prime Minister's deal. 'As the EU makes absolutely clear, nothing can be offered which changes or is inconsistent with the Withdrawal Agreement.
'Nothing has changed. That confirms this deal is the worst of all worlds, transforming the UK into a long-term rule taker.'
The letter comes as the PM launched a final bid to win round MPs to her Brexit deal today - warning that voting it down will probably mean staying in the EU.
The Prime Minister is urging Tory Eurosceptics to look at the mounting revolt by Remainers, and realise that Parliament is ready to block the country from crashing out.
In a speech in Stoke on Trent just 36 hours before the titanic Commons showdown, Mrs May pleaded with MPs to consider the 'consequences' of their actions for people's faith in democracy.
She said the assurances published by the EU today will provide her backbenchers with the 'further clarifications and further assurances' they need to back her deal.
The PM's desperate scramble comes as tensions escalate in Westminster ahead of the vote on the Brexit package she has thrashed out with Brussels.
Mrs May looks to be on track for a catastrophic defeat - with frantic maneuvering under way over what happens next.
A dozen Tory former ministers including Boris Johnson have urged wavering colleagues to stand firm against the deal, saying leaving on World Trade Organisation terms would not be a disaster.
But Remainers from across parties are plotting an extraordinary bid to seize control from the government if it tries to push ahead with a no-deal Brexit.
Conservative MP Nick Boles today confirmed plans to tear up Commons rules so MPs could propose legislation - something the government currently has power over.
In a speech in Stoke on Trent just 36 hours before the titanic Commons showdown, Theresa May (pictured in Stoke on Trent today) pleaded with MPs to consider the 'consequences' of their actions for people's faith in democracy
Ministers fear Speaker John Bercow would help the rebellion. Last week he flouted procedural convention to select an amendment from Tory former minister Dominic Grieve which attempts to speed up the process for the Government to reveal what it will do next if the PM's Brexit deal is rejected.
Speaking in Stoke-on-Trent, Mrs May warned MPs would be behaving recklessly if they did not back her Withdrawal Deal.
She said: 'The only ways to guarantee we do not leave without a deal are to abandon Brexit, betraying the vote of the British people, or to leave with a deal.
'The only deal on the table is the one MPs will vote on tomorrow night.
'You can take no deal off the table by voting for that deal.
'If no deal is as bad as you believe it is, it will be the height of recklessness to do anything else.'
A dozen Tory former ministers including Boris Johnson (pictured in London today) have urged wavering colleagues to stand firm against the deal
No10 believes Sir Oliver Letwin (pictured left) and Dominic Grieve are the main figures behind the plot to overhaul Commons rules
But as she pleaded for unity, Tory rebels leaders were urging their fellow Conservatives to vote the deal down.
Asked during an LBC radio interview today what would happen tomorrow, Boris Johnson said: 'I think the deal goes down.
'I think, possibly, some colleagues are being scared by this idea that there might be no Brexit as a result of voting it down.
'I think that's nonsense. 'Britain will leave in March, absolutely, and that's the bottom line.'
A dozen leading Brexiteers – including eight former members of the PM's Cabinet – have written to all Conservative MPs urging them to vote against her deal.
Former ministers including Mr Johnson, David Davis and Dominic Raab call upon Mrs May to stage one final attempt to persuade the EU to drop the Irish backstop which threatens to halt Britain's exit from the custom union indefinitely.
But if the EU fails to comply on agreeing such a deal, the Britain must 'have the confidence' to leave on World Trade Organisation (WTO) terms on March 29.
On the BBC's Andrew Marr show yesterday, Jeremy Corbyn made clear the party is on high alert to try to force the PM out and a general election
The letter is also signed by other former Cabinet members including Iain Duncan Smith, Esther McVey and Priti Patel.
They write: 'It is right to vote down this bad deal and that in doing so we will unlock a better future for our party, our country and its people.'
They add: 'A managed WTO Brexit may give rise to some short-term inconvenience and disruption, but the much greater risks arise from being locked into a very bad deal.'
Boris Johnson repeats the message in his column in the Daily Telegraph today, where he writes: 'This deal is still the worst of both worlds, by which we somehow leave the EU but end up being run by the EU. It is still a complete stinker.'
Warning his fellow MPs about trying to force a second referendum, he adds: 'If they now engage in ludicrous parliamentary jiggery pokery, endlessly tabling amendments designed to frustrate Brexit, they will risk a very serious backlash indeed.
'The answer is not to leave it to Parliament; the answer is for the executive to do its job, as some of us have been advising for months: to accept that the deal is dead, and to move on.'