Parliament is sitting on a Saturday for the first time in 37 years to vote on Boris Johnson’s Brexit deal.
The PM is 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.
Mr Johnson told MPs “now is the time to get this thing done”, saying any delay beyond 31 October would be “corrosive”.
But MPs will debate a proposal that could delay Brexit again until all the necessary UK legislation is passed.
Commons Speaker John Bercow has chosen an amendment that, if passed, would require the PM to ask for a further extension to the Brexit process by the end of Saturday.
With the PM’s former DUP allies and opposition parties planning to vote against the PM’s deal, the vote is expected to be incredibly close.
At least nine Labour MPs are expected to 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.
MPs have assembled for the first weekend sitting since the invasion of the Falklands in 1982.
After making a statement, Mr Johnson will face MPs’ questions before the House moves on to a debate about the deal, with votes not expected before 14.30 BST.
Opening the debate, Mr Johnson urged MPs to come together to begin to “heal the rifts in British politics”.
He said a majority of MPs were committed to delivering the result of the 2016 referendum and urged opponents to “abandon their delusion” that any further delay would help the UK.
“It is my judgement we have reached the best possible solution,” he said. “In this crucial mission, there cannot be longer any argument for further delay.”
Letwin amendment
Among the amendments chosen by Commons Speaker John Bercow is a controversial one put down by former Tory Sir Oliver Letwin, who now sits as an independent.
This would withhold parliamentary support for the deal unless and until legislation implementing the agreement in UK law is passed by MPs.
If this amendment 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.
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.
Mr Johnson has repeatedly said Brexit will happen by the end of the month with or without a deal.
However, MPs passed a law in September, known as the Benn Act, which requires the PM to send a letter to the EU asking for an extension until January 2020 if a deal is not agreed – or if MPs do not back a no-deal Brexit.