Number 10 has launched a frantic operation to convince a majority of MPs to support Boris Johnson’s Brexit deal in a knife-edge vote – while rebels are looking to block an accidental no-deal.
An amendment proposed by Sir Oliver Letwin could see MPs withhold support of Mr Johnson's deal until the Brexit legislation has been safely approved.
If selected by Commons Speaker John Bercow later today, the Letwin amendment would trigger the Benn Act introduced by MPs opposed to a no-deal, and it would compel the PM to request a delay until the end of January.
Sir Oliver has said he wants to make sure Britain does not leave without a deal by mistake.
"My aim is to ensure that Boris' deal succeeds, but that we have an insurance policy which prevents the UK from crashing out on 31 October by mistake if something goes wrong during the passage of the implementing legislation," he said.
He explained to Sky News that his amendment "does not have any effect on the actual important vote, which is if we support this deal or don't".
He added: "I do support this deal, I will vote in favour of this deal because I think it is a very sensible deal.
"The amendment is to make sure that we have an insurance policy that means if for some reason something goes wrong when the legislation goes through, then we would have already secured the extension which the Benn Act provides, so we won't crash out of the EU by mistake."
Sky News' deputy political editor Sam Coates said: "There are so many variables. It does look as if this will pass, it does look as if Mr Johnson will be obliged to write that letter on Saturday evening, and then where will be be next week? What happens if parliament does approve the legislation? What cards does Mr Johnson have to play? What will the opposition do?"
Meanwhile, Number 10 is trying to secure passage of the deal.
Tory Brexiteer MPs who have previously refused to back a withdrawal agreement were spotted entering Downing Street for talks on Friday.
Number 10 officials are also understood to be holding discussions with Labour MPs who might back the deal - but Labour leader Jeremy Corbyn has urged his party members to vote against it.
Sky News political correspondent Lewis Goodall said there has been a "huge" whipping operation, adding: "Never underestimate the Labour whips."
The prime minister, speaking to the BBC, said: "Clearly, what matters is MPs coming together across the house to get this done.
"And I just kind of invite everybody to imagine what it would be like if we have settled this and we have respected the will of the people because we will then have a chance to move on."
French President Emmanuel Macron upped the stakes ahead of Saturday's critical vote for those MPs who want to avoid a no-deal Brexit, as he voiced his opposition to a further extension to the Article 50 negotiating period.
He said: "I wish that we can finish this off and speak about the future. The 31 October date should be respected. I don't believe new delays should be granted."
If Mr Johnson does not secure support for his Brexit deal, he will be compelled by law to seek a three-month Article 50 extension from EU leaders.
However, a further Brexit delay is at the discretion of the bloc and could be vetoed by member states.
Mr Macron's comments will help the prime minister portray today's vote as a choice between a deal or a no-deal Brexit.
A handful of MPs, from both the Conservatives and Labour, who refused to back his predecessor Theresa May's withdrawal agreement have already said they will support Mr Johnson's deal.
But opposition MPs have set out their plans to derail Mr Johnson's vow to deliver Brexit by 31 October "do or die".
The outcome remains in the balance, especially with the DUP pledging to vote against the deal.
Labour, the Liberal Democrats and the SNP are all opposed, meaning the parliamentary arithmetic is looking tight.
The PM needs 320 votes to pass an agreement, presuming every MP votes.
Sky News analysis suggests that - as it stands - Mr Johnson has a path to gaining about 316 votes, so the outcome is on a knife edge.
Speaking in Brussels after EU leaders signed off a deal on Thursday night, the PM was typically optimistic.
Mr Johnson said he was "very confident" MPs would approve the deal, calling on parliamentarians to "come together" and deliver Britain's exit from the bloc in time for the 31 October deadline.
"We've been at this now, as I say, for three-and-a-half years," the PM said.
"It hasn't always been an easy experience for the UK. It's been long, it's been painful, it's been divisive.
"And now is the moment for us as a country to come together.
"Now is the moment for our parliamentarians to come together and get this thing done."
Asked about the DUP's opposition, Mr Johnson claimed the agreement was a "good deal for every part of the UK, particularly Northern Ireland".
Pressed by Sky's political editor Beth Rigby as to whether he was making the same mistake as Mrs May by not getting the DUP on side, the PM repeated his optimistic message and declared: "The opportunity is great, let's do it."
But the DUP's Westminster leader, Nigel Dodds, said Mr Johnson had been "far too eager to get a deal at any cost".
He said: "The fact of the matter is, if he held his nerve and held out he would, of course, have got better concessions that kept the integrity, both economic and constitutionally, of the United Kingdom."
The party said the agreement created several economic borders down the Irish Sea, which separated Northern Ireland from the rest of the UK.
In comments that will be seen as putting pressure on MPs to back the PM's deal, the EU's Jean-Claude Juncker said there was "no need for prolongation" of the Brexit process as there was a deal on the table.
"That's not only the British view, that's my view too," the outgoing European Commission president said.
Another senior EU figure, European Council President Donald Tusk, voiced his "sadness" at the UK's exit and said "our door will always be open" if it wants to rejoin.
This was echoed by Irish Prime Minister Leo Varadkar, who described the UK as "like an old friend that is going on a journey without us".
He said there would "always be a place" if it decided to return.
As attention turns back to Westminster, focus will in part turn to the call in some quarters for a second referendum on the deal the PM has struck.
Jeremy Corbyn has said he does not "suspect" the option of holding a vote in parliament on whether to have a referendum would arise today.
He described reports that his party could back such a move as "high-level speculation on a hypothetical question".
Meanwhile, the SNP's Westminster leader Ian Blackford has tabled an amendment to the "appalling" deal, demanding an immediate extension and general election.
If the PM fails, then under the terms of legislation passed by opposition MPs he will have to ask for a delay to Brexit in order to avoid a no-deal exit.
A senior government source said on Thursday that the PM's position was "new deal or no deal, but no delay".