Theresa May suffered another humiliating defeat tonight as Tory rebels forced through an amendment giving Parliament more power over policy.
The House of Commons passed an amendment making clear that it should be able to direct the government how to respond if the PM loses the crunch vote on her Brexit deal.
The stinging setback came minutes after the government was found in contempt of the Parliament for the first time in history - and moments before Mrs May took to her feet to make the case for the package she has thrashed out with the EU.
Kicking off a marathon five days of debate, Mrs May insisted her blueprint was the only way to 'bring the country back together'.
She pleaded with MPs to recognise 'life depends on compromise, and warned that killing off her deal would risk not 'delivering Brexit' - something she said would be disastrous for the public's faith in politics.
But the premier was greeted with jeers taunts from the Opposition benches, while Tory allies looked on glumly.
Leader of the House Andrea Leadsom urged MPs not to 'preempt' the result of next week's crunch vote on the EU deal by agreeing that the Commons should be able to direct the government how to respond if they lose
A group led by Tory rebel Dominic Grieve (pictured) tabled an amendment to the PM's deal that could give MPs power to direct the government
Commons Leader Andrea Leadsom had appealed for MPs not to endorse the amendment, but it was passed by 321 to 299.
Tory rebel Dominic Grieve insisted the change was 'sensible' and accused ministers of 'disrespecting' Parliament by trying to block it.
Labour also signalled strong support for the amendment, with shadow Commons leader Valerie Vaz jibing that any 'true democrat' would back it.
The rebels are trying to to ambush the government by pushing the change to a vote this evening - before the marathon Brexit debate gets formally under way.
Firms urged to register for export licences as no-deal Brexit planning escalates
Ministers are urging 145,000 firms to register for new export licences as no-deal planning escalates.
Companies which trade with the EU are being contacted to say they have to register for a UK economic operator registration and identification number (EORI).
HM Revenue and Customs will be contacting 145,000 businesses who only import or export goods with the EU to advise them to register for an EORI.
The Prime Minister's spokesman said: 'If you are a business which exports to the rest of the world you will currently have one of those, they are an HMRC requirement; if you only do business within the EU, you may well not have one of those.
'HMRC is today advising that you will need one in able to continue to make declarations in the future.'
The package thrashed out by Mrs May with Brussels is due to come before the Commons for a final vote on December 11, in a showdown that could define the country's future for generations.
But Mrs May is staring down the barrel of a catastrophic defeat as around 100 Tories have publicly signalled opposition, while Labour, the SNP and the Lib Dems are also against.
Ministers have been trying to reduce the potential for Parliament to undermine the government's approach by restricting amendments to their plan.
If the PM loses the so-called 'meaningful vote' on December 11, the EU Withdrawal Act states that it must come back to the Commons within 21 days and spell out their intentions.
However, under normal Standing Orders at that point the plan would only be subject to a 'neutral motion' - meaning MPs could only 'take note' of the proposed approach.
The rebel amendment, endorsed by more than a dozen MPs including Mr Grieve, former Cabinet Office minister Oliver Letwin, and Treasury Committee chair Nicky Morgan, would remove this restriction for when the Commons comes to consider the government next steps.
That would allow MPs to table amendments that would reject the government's plans, and instruct ministers to do something different - such as extending Article 50 and holding a second referendum.
While not necessarily binding, such an expression of Parliament's will would have huge political force and might well be impossible to ignore.
By attaching the amendment to the 'programme motion' for the Brexit deal vote - which sets out the timetable - Mr Grieve could ensure it it voted on tonight, rather than next Tuesday with other amendments.
The Speaker has yet to decide which amendments to select, although most observers expect him to choose Mr Grieve's.
The measure is very similar to a motion tabled Labour's Hilary Benn - but is more likely to attract support from Tories.
Brexiteers tried to play down the importance of the change tonight.
Former Brexit minister Steve Baker said: 'Grieve's Amendment D if passed allows for an amendable motion 21 days after a Government defeat of their dreadful deal
'Whatever the outcome of the amendment, it is not legally binding on the PM. Acts are law, motions are motions. The executive still decides how to proceed.'
If the PM loses the so-called 'meaningful vote' on December 11, the EU Withdrawal Act states that it must come back to the Commons within 21 days and spell out their intentions