Parliament is sitting on a Saturday for the first time in 37 years to vote on Boris Johnson's Brexit deal.
The PM has been trying to convince MPs to support the agreement he secured with the EU, ahead of what is expected to be a knife-edge vote in the Commons.
His former DUP allies and opposition parties plan to vote against it.
Brexit Secretary Stephen Barclay admitted the vote could be "close" but said the government has "listened to the concerns of MPs across all sides".
"Now it's time for MPs to step up to their responsibility to get this deal passed, and allow the country to move forward," he told BBC Breakfast.
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At least nine Labour MPs are expected support the government while the PM is hoping to be backed by some of the 21 Tory MPs he sacked for opposing him last month.
Steve Baker, the chairman of the European Research Group, a group of Tory Brexiteers, recommended its members vote in favour of the deal at a meeting on Saturday morning.
Business in the House of Commons will start at 9:30 BST - the first weekend sitting since the invasion of the Falklands in 1982.
Mr Johnson will make a statement to MPs and face their questions before the House moves on to a debate about the deal.
The timing of any votes depends on which amendments are chosen by the Speaker of the Commons, John Bercow, but they are not expected before 14:30.
The most significant and controversial amendment is one put down by former Tory Sir Oliver Letwin, who now sits as an independent, which would withhold parliamentary support for the deal unless and until legislation implementing the agreement in UK law is passed by MPs.
If this is passed, it would force the prime minister to seek a further delay to Brexit beyond the 31 October deadline - under the terms of the Benn Act passed last month.
The BBC's political editor Laura Kuenssberg said its "brutal effect" would be to deny the PM the opportunity of having the "conclusive" vote on his deal he so badly wants.
A No 10 source has said the vote on Mr Johnson's deal will be abandoned if the amendment is passed, saying it will "render the entire day meaningless".
"A vote for Letwin is a vote for delay and the whips will send everyone home," they said. "It would perfectly sum up this broken Parliament."
Former Tory Chancellor Philip Hammond, who is backing the amendment, told the BBC it was an "insurance policy" to ensure the UK did not leave the EU later this month without a deal if the necessary legislation was not passed in time or was scuppered by MPs.
"This cannot be the final vote because we don't know the full shape of the package," he told BBC Radio 4's Today.
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He insisted he was not trying to stop Brexit and it might only lead to a delay to the UK's departure of a matter of weeks.
Source:BBC