The Prime Minister looks set for humiliating defeat in the “meaningful vote” taking place in the House of Commons at the end of eight days of debate and two years of negotiation following the EU referendum of June 2016.
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Rejection of Mrs May’s Withdrawal Agreement by MPs would give her until January 21 to set out her Plan B - expected to involve going back to Brussels to seek further concessions.
And it is also likely to trigger a bid to force a general election by Jeremy Corbyn, who has said he will table a motion of no-confidence in the Government “soon” after it is defeated on its central policy platform.
Moments before the crunch vote, Mrs May told MPs: “Parliament gave the people a choice, we set the clock ticking on our departure and tonight we will determine whether we move forward with a Withdrawal Agreement that honours the vote and sets us on course for a better future.
“The responsibility of each and every one of us at this moment is profound, for this is a historic decision that will set the future of our country for generations.”
But the Labour leader called on MPs to vote down the agreement, saying: “This deal is bad for our economy, a bad deal for our democracy, and a bad deal for this country.”
Mr Corbyn could use a point of order in the immediate wake of Tuesday’s vote to trigger a no-confidence debate as early as Wednesday.
And Mrs May is expected to deliver her immediate response to the reverse in a statement to the Commons moments after her anticipated drubbing.
Attorney General Geoffrey Cox appeared to indicate that the PM will resist pressure to tear up her plan or to seek cross-party consensus on a new approach.
He told MPs that in the event of a Government defeat the Agreement would have to return to the Commons later “in much the same form with much the same content”.
Noisy crowds of pro- and anti-Brexit protesters in Parliament Square could be heard inside the Palace of Westminster as MPs prepared to vote at 7pm, in a process expected to culminate in a final result around 7.30pm.
MPs were due to vote first on a series of four amendments chosen by Speaker John Bercow.
But Mr Corbyn, Scottish National Party leader Ian Blackford and Tory backbencher Sir Edward Leigh opted not to move their amendments, leaving only one division on a proposal from Conservative MP John Baron for the UK to take unilateral powers to end controversial “backstop” arrangements.
Heavily pregnant Labour MP Tulip Siddiq, who postponed her planned Caesarean in order to vote, attended the Commons in a wheelchair.
By the time Mrs May concluded the debate with a passionate plea to MPs to deliver the Brexit demanded by voters, her hopes of victory appeared vanishingly small.
Assurances over the “backstop” received from the EU on Monday appear to have failed to win over significant numbers of the deal’s critics among the Tories and their DUP allies.
While some MPs - including former Labour minister Frank Field - declared their intention to back the PM’s deal, normally loyal Tories like Sir Hugo Swire said they would not do so.
Confirming that the 10 DUP MPs will not back the PM’s deal, party leader Arlene Foster told a press conference in London: “We said to the Prime Minister she had to get rid of the backstop and get a Withdrawal Agreement that can be lived with. I don’t think she even asked to get rid of the backstop.”
In the final day of debate on Mrs May’s deal, Mr Cox acknowledged that the agreement reached with the EU in November was not perfect, but said he was supporting it “for wholly pragmatic reasons”.
“It is the necessary means to secure our orderly departure and unlock our future outside the European Union,” he told MPs.
Thousands of businesses with contracts based on EU law would find “the rug pulled from under (them)” if the UK left with a deal, he said.
They would ask MPs: “What are you playing at? What are you doing? You are not children in the playground, you are legislators’,” Mr Cox said, adding: “We are playing with people’s lives.”
Mr Cox compared the Withdrawal Agreement to an “airlock” providing a route towards a future trade and security relationship with the remaining 27 EU states.
But the Labour chair of the Brexit Select Committee Hilary Benn retorted: “The reason why many of us will vote against the deal tonight is because on the other side of the second airlock is a complete vacuum about our future relationship with our biggest, nearest and most important trading partners.”
Mrs May’s deal came under assault from a series of MPs on both sides of the Brexit debate, with eurosceptics warning that it failed to resolve their concerns that the backstop arrangements designed to avoid a hard border in Ireland could become permanent.
The Tory chairman of the European Scrutiny Committee Sir Bill Cash called on Mrs May to “consider her position”, comparing it to Neville Chamberlain’s resignation in 1940 which cleared the way for Sir Winston Churchill to take over.
“Now is the time to walk away from the intransigence of the EU and our failed policy of seeking to supplicate their guidelines, their terms and their paymasters,” he said.
In the final Cabinet meeting before the crucial vote, Mrs May told senior ministers that the Government was “the servant of the people” and she believed “passionately” that it must deliver on the result of the referendum, in which voters opted to Leave by a margin of 52%-48%.
She spent much of the day meeting Tory MPs in her Commons office in an eleventh-hour bid to bolster support.
But analysis of polling released by the Best for Britain campaign for a second referendum suggested that around 60% of voters - including majorities in every region of the country - want a public vote if Parliament proves unable to decide on a Brexit deal.
German foreign minister Heiko Maas said Brexit talks would continue if Mrs May’s deal was rejected by MPs, but there was unlikely to be “substantial” change to the agreement.
Mr Maas said: “If there were still a solution that could be presented under even greater pressure, I would ask myself why it has not been put on the table before in order to ensure that this evening’s vote takes place under better circumstances.
“That is why I believe that the agreement is as it stands and will not be substantially changed, but that, if things go wrong tonight, there will certainly be talks again.”