Tory rebels have promised to wage a parliamentary guerrilla campaign to prevent a no-deal Brexit if Theresa May’s plan for EU withdrawal is defeated in the Commons.
The House will begin five days of debate on Wednesday, culminating in the “meaningful vote” next week which could determine the fate of the government.
It comes after the prime minister dramatically pulled a vote before Christmas, admitting she was heading for defeat in the face of opposition from both pro-Leave and pro-Remain Tories.
Labour has said it will table a motion of no confidence in the government should Mrs May lost the vote on January 15.
Ahead of the debate, MPs opposed to a no-deal break were flexing their muscles on Tuesday evening to inflict another bruising defeat on the Government.
Twenty Conservative MPs joined opposition parties in backing a cross-party amendment to the Finance Bill intended to limit the Government’s powers to make tax changes in the event of no-deal.
The rebels included former ministers Ken Clarke, Sir Michael Fallon, Justine Greening, Dominic Grieve, Sir Oliver Letwin and Sam Gyimah.
Sir Oliver, who supports Mrs May’s deal, said they were ready to table similar amendments to other Brexit legislation to warn they were prepared to put paid to “this disastrous proposal”.
“The majority in this House will sustain itself, and we will not allow a no-deal exit to occur on March 29,” he said.
Downing Street insisted the amendment – tabled by senior Labour MP Yvette Cooper and Tory former cabinet minister Nicky Morgan – was no more than an “inconvenience” which would not prevent the government collecting taxes.
But supporters said the vote – of 303 to 296 – showed there was now a clear majority of MPs who would oppose a no-deal if Mrs May cannot, as many expect, win the backing of the Commons for her agreement.
The defeat marked the first time a government has lost a vote on the Finance Bill since 1978.
Among Brexiteers there was some suspicion as to why ministers chose to oppose a relatively minor amendment rather than avoid defeat by letting it go through unopposed.
Steve Baker, a leading member of the pro Brexit European Research Group, suggested it was a “whipping tactic” to “deliberately advertise weakness” in the hope of persuading pro-Brexit MPs to back the deal rather than risk remaining in the EU.
Shadow international trade secretary Barry Gardiner said Labour would table a motion of no confidence in the government if Theresa May’s Brexit deal is voted down next week.
“We’re now talking as if it is expected that the government next Tuesday will be defeated on the most important piece of legislation that has come before Parliament probably in 50 or more years, that the government has devoted two years of its time to try to negotiate, and we’re now almost accepting that this will simply be defeated and voted down,” he told BBC Radio 4’s Today programme.
“Obviously, the next thing to do immediately after that is for there to be a vote of confidence in the government.”
Mr Gardiner said the “appropriate time” to table such a motion is when the “government loses its key legislation and no longer commands a majority in the House of Commons”.
The prime minister has said she is still seeking fresh assurances from the EU on the Northern Ireland backstop, intended to prevent the return of a hard border with the Republic, in a bid to win over sceptical MPs.
Meanwhile, a cross-party group of pro-EU MPs have backed a new amendment that would require Mrs May to return to the Commons with a new plan within just three days if she loses the meaningful vote.
Current legislation says that if the vote is lost, ministers have 21 days to make a statement to the Commons, followed by seven days in which to table a fresh motion, a total of 28 days.
The amendment by Tory former attorney general and Brexit rebel Mr Grieve seeks to drastically shorten that time to allow an alternative to be found that heads off a no-deal Brexit.
It is backed by Tories including Sir Oliver and Jo Johnson, plus Labour’s Chris Leslie among others.
Downing Street had been hoping that MPs would come under pressure from their constituencies over the Christmas break to support the deal rather than risk the consequences of a disorderly no-deal break.
But with the EU continuing to insist there could be no further negotiations, there was little sign that the parliamentary arithmetic had shifted significantly over the holiday period.
Labour said the wording of the Government motion confirmed it would be a “continuation debate” and that MPs were being asked to vote on the exact same deal as before.
Party leader Jeremy Corbyn said: “The delay to the meaningful vote has achieved nothing beyond wasting a month.
“If the government’s deal is defeated then a general election is the best way forward to give a new government a mandate.”
Brexit secretary Stephen Barclay will open the debate for the Government, with Mrs May herself due to wrap up five days of discussions in the Commons with a final appeal to MPs to vote for her deal on the evening of Tuesday January 15.