Theresa May is making a final bid to save her
Brexit deal ahead of a crunch Commons vote.
Battling a sore throat after late night talks with the EU, the prime minister urged MPs to back her "improved deal" or risk "no Brexit at all".
But a key group of Tory Brexiteers and the DUP, who keep her government in power, have both rejected the deal.
They say the legal assurances secured by the PM are not enough to prevent the UK being tied permanently to the EU.
In a statement, the
European Research Group - headed by Tory MP Jacob Rees-Mogg - said: "In the light of our own legal analysis and others we do not recommend accepting the government's motion today."
The DUP said in a statement "that sufficient progress has not been achieved at this time".
With husband Philip watching from the public gallery, Mrs May told MPs that "Brexit could be lost" if her deal is rejected.
"The danger for those of us who want to have faith in the British public and deliver on their vote for Brexit, is that if this vote is not passed tonight, if this deal is not passed, then Brexit could be lost."
Charles Walker, vice chairman of the 1922 Committee of backbench Tory MPs, said defeat for the government in the vote later would lead to a general election.
He told BBC Radio 4's World At One: "If it doesn't go through tonight, as sure as night follows day, there will be a general election within a matter of days or weeks.
"It is not sustainable, the current situation in Parliament."
He added: "She will lead us into that general election. We haven't got time for a leadership election."
It comes after Attorney General
Geoffrey Cox told MPs the legal risk of being tied to the EU after Brexit "remains unchanged".
He added that the new assurances secured by the PM did "reduce the risk that the United Kingdom could be indefinitely and involuntarily detained" in the Irish backstop if talks on the two sides future relationship broke down due to "bad faith" by the EU
He defined "bad faith" as a "pattern of refusing to accept reasonable proposals" on the Irish backstop.
But he said the question of whether a satisfactory post-Brexit deal on a permanent trading relationship can be reached remained "a political judgment" - and he said MPs should back the PM's deal.
In his advice, Attorney General Geoffrey Cox said "the legal risk remains unchanged" that if a post-Brexit trade agreement can not be reached due to "intractable differences", the UK would have "no internationally lawful means" of leaving the backstop without EU agreement.
More reaction from MPsThe last time Mrs May's withdrawal agreement was put to Parliament in January, it was voted down by a margin of 230.
The BBC's political editor Laura Kuenssberg said it would be a "political miracle of historic proportions" if Mrs May could overturn such a heavy defeat.
Mrs May earlier addressed a meeting of Conservative MPs, in an effort to change the minds of those opposed to her deal.
Conservative MPs leaving the meeting suggested half of those who voted against deal last time will switch to support it later, said BBC's Chief Political Correspondent Vicky Young.
Former minister Grant Shapps said the vote would be close and Mrs May "needed the DUP" to back her deal.
Work and Pensions Secretary Amber Rudd said she believed the prime minister's deal would go through "otherwise instability will follow which would be so unwelcome".
Labour's Shadow Brexit Secretary Sir Keir Starmer said: "The government's strategy is now in tatters."
What was agreed with the EU?Documents were agreed after Mrs May flew to the European Parliament with Brexit Secretary Steve Barclay for last-minute talks with Mr Juncker and EU chief Brexit negotiator Michel Barnier.
- A "joint legally binding instrument" on the withdrawal agreement which the UK could use to start a "formal dispute" against the EU if it tried to keep the UK tied into the backstop indefinitely.
- A joint statement about the UK and EU's future relationship which commits to replacing the backstop with an alternative by December 2020.
- A "unilateral declaration". stating there is nothing to stop the UK from leaving the backstop if discussions on a future relationship with the EU break down and there is no prospect of an agreement.
Many MPs fear the backstop, initially agreed by the UK government in December 2017, would keep the country in a customs arrangement with the EU indefinitely.
The PM has claimed the new documents addresses this issue and urged MPs to back the "improved deal".
The EU warns 'this is it'
The UK is set to leave the EU on 29 March 2019 after voting to leave by nearly 52% to 48% - 17.4m votes to 16.1m - in 2016.
Mr Juncker has warned MPs they would be putting everything at risk if they voted down the deal.
"In politics sometimes you get a second chance," he said. "It is what we do with that second chance that counts. There will be no third chance."
The Irish Prime Minister Leo Varadkar said the new agreements showed both sides' "good faith" - although he made clear they did "not undermine" the principle of the backstop or how it might come into force.