MPs are preparing to vote on whether to return Theresa Mays agreement to leave the European Union.
The so-called "meaningful vote" takes place later, when five-day debate on Brexit is completed.
Mrs May has called on politicians to reverse their agreement or risk "letting British people fall down".
But with many of his own parliamentarians, who are expected to join the opposition parties to vote against the agreement, it is generally expected to be defeated.
Advocate General Geoffrey Cox opens the last day of the debate at. 1
Voting starts at. 19.00 GMT, starting with amendments that can reformulate the agreement and then the vote on the withdrawal agreement itself.
The Prime Minister speaks Tuesday morning after she tried Monday night to win Tory MEPs' support for her deal – which includes both the withdrawal agreement on the terms that the UK leaves the EU and a political statement for the future.
In Com mons, she said: "It's not perfect, but when history books are written, people will look at Parliament's resolution and ask:" Should we hand over the country's vote to leave the EU, we protected our economy, security or union , or do we let the British people down? & # 39; "
Mrs May has also tried to assure MEPs of the controversial Northern Ireland" backstop "return plan to avoid any return to physical border control between the country and Ireland.
She pointed to new written assurances from the EU on that the unforeseen customs arrangements proposed would be temporary and, if triggered, would be "the shortest possible period."
Environmental Secretary Michael Gove told BBC Radio 4's Today program that rejection of Mrs Mays agreement would lead to a non-traded Brexit with short-term financial damage "or worse, no Brexit at all. "
He said with this agreement:" We have picked up a whole bowl of glossy cherries ", despite the fact that the EU had said at the beginning of the negotiations that there would be" cherry picking. "" If we do not vote for this deal, we risk playing in the hands of those who do not want Brexit to continue, "he said.
But many Tory MPs and democratic unionists remain fiercely against the agreement
About 100 conservatives MEPs – and the 10 MEPs of the Democratic Unionist Party – could join Labor and the other opposition parties to vote it. Former Brexit Secretary Dominic Raab said that Brexiteers as he could reverse an agreement if such aspects as backstop were dealt with
He told today's program that the EU had played "a smart game with hard ball" and said it was time for the UK to do the same.
Democratic Unionist leader Arlene Foster called backstop & # 39; a toxic & # 39, and said that the 10 parliamentarians would vote against the deal.
"It's time for a sensible deal that steers our exit from the EU and supports all parts of the UK" she said on Twitter .
The deal suffered a heavy defeat at the House of Lords on Monday night, when peers supported a Labor Movement with 321 votes to 152.
While the vote has no real weight, as MEPs whom Members of Parliament should have the final say, the proposal – which also rejected a "no deal" scenario – said "regret" that Mrs Mays agreement would "damage the future economic prosperity , internal security and global influence "in the UK.
However, five conservative British MPs who have been critics of the withdrawal agreement have now said they will support the government along with three workers and self-employed Frank Field.
Brexit Secretary Steve Barclay said it turned out that there was "progress", but admitted BBC's Politics Live, which was supported was "challenging."
A number of amendments to Mrs Mays agreement have been tabled by MEPs to try to change it in Parliament.
The proposals include giving parliamentarians a vote on whether to carry backstop and setting a time limit on how long backstop could last.
Minister of Labor Hilary Benn had planned an amendment to reject the agreement and prevent any agreement – but has since told the BBC's political editor Norman Smith that he has withdrawn his proposal.
Benn told the Today program that he wanted a "clear, single voice" on Mrs Mays agreement, so there was "clarity" on why it was rejected.
When asked what the margin of defeat could be for Mrs May, former downing street director of legislative affairs told Nikki da Costa today that she expected to be within the "50 to 80 mark".  Commons Speaker, John Bercow, will decide which changes can continue to be voted on just before the vote on the actual deal.
Told Mr. Corbyn again the agreement and repeated his call for a choice if it was reconciled by Parliament. He also promised that Labor would call a vote of confidence in the government "soon".
He said: "Theresa May has tried to extort workers' parliamentarians to vote for her damned agreement by threatening the country with chaos without agreement. I know from conversations with colleagues that this has failed. The Labor Party will not be held for ransom. "
At the end of seven-hour Commons debate, Shadow Chancellor John McDonnell, whose Work could not force a change of government, said ministers should give power to allow parliamentarians across parliament to work together" to ensure the best compromise to protect our country. "
Chancellor Philip Hammond settled the fourth debate day just after midnight. 02:00 GMT on Tuesday by warning that no one would get "exactly the Brexit they want".
If you leave the EU without an agreement, it would be "just as much betrayal as not at all Brexit," he argued, saying that it would not deliver the promise of greater prosperity.
What happens later?
If the agreement is rejected by MPs, Mrs May has three days to return to Parliament with a "Plan B".
Some have suggested that she would head to Brussels on Wednesday to try to get further concessions from the EU before returning to the public to make a statement on her new proposal on Monday. This could then be reconciled by MPs.
If this also fails, there is a proposal made by senior conservative backbenchers Nick Boles, Sir Oliver Letwin and Nicky Morgan for an "EU Extension No 2 Bill". This would allow ministers another three weeks to come up with another plan and get it through parliament.
If this does not work, they propose to assume responsibility for reaching a compromise agreement for the liaison committee – consisting of the presidents and chairmen of all the committees selected, also elected by opposition parties such as the conservatives.
This proposal will again be reconciled by MEPs.
- Laura Kuenssberg explains some of the many potential scenarios after the vote
New referendum proposal
In another development, a cross-segment group of anti-Brexit politicians has published legislative proposals to create a referendum to ask the public whether they will remain in the EU or leave under the Prime Minister's agreement.
MEPs behind the draft law point out that Article 50 – the two-year process whereby an EU member can leave the block – must be renewed for another referendum to take place, which means that the United Kingdom remains a member outside of March 29.
But unless new legislation is introduced, the default position will be that the UK leaves the EU on that date without agreement.