Boris Johnson has said he is "very confident" MPs will back his last-minute Brexit deal despite the DUP and Labour ruling out support.
The PM urged Parliamentarians to "come together and get this thing done" after EU leaders unanimously approved the departure agreement hammered out shortly before an EU summit on Thursday.
It means the newly won agreement now must be approved by MPs in Parliament on Saturday before the UK is able to leave the EU with a deal on October 31.
However, if MPs reject the Prime Minister's Brexit offer, it is likely he will be forced to send a letter to the EU requesting an extension past the deadline.
But the DUP, SNP, Labour and the Liberal Democrats have already signalled they will not support the deal, meaning a tense day of voting on Saturday - the first weekend sitting of Parliament since the Falklands War.
EU Commission president Jean-Claude Juncker piled the pressure on MPs to back the deal in the parliamentary showdown by raising doubts over any further delay to the UK's departure past October 31.
Should the deal pass through Parliament, it will still need to be ratified by the European Parliament but is expected to succeed.
However, the European Parliament's chief Brexit official said this may not happen before Mr Johnson's "do or die" deadline of Halloween, as MEPs will need to carefully examine the deal before making a decision on it.
Guy Verhofstadt said MEPs will only start their work once the UK Parliament has passed a fully binding Brexit deal.
If that slips past the European plenary session next week, it could well have to be picked up in the session that begins on November 13.
But before the deal can even come before MEPs it needs to pass MPs' approval, and many are already stating they will not back it.
Speaking at a press conference in Brussels, Mr Johnson appealed to all MPs to work across party lines to support his deal.
"I am very confident that when my colleagues in Parliament study this agreement that they will want to vote for it on Saturday and in succeeding days," he said.
"We've been at this now, as I say, for three and a half years.
"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."
Appealing to Arlene Foster's DUP party, he insisted the country can leave the bloc "as one United Kingdom" and "decide our future together".
However, earlier in the day DUP deputy leader Nigel Dodds took aim at Mr Johnson's negotiation strategy, saying he had been "too eager" to win a deal.
The party added it would be "unable to support these proposals in Parliament".
He said: "In order to avoid an extension he has been too eager by far to get a deal at any cost.
"The fact of the matter is if he had held his nerve and held out he would of course got better concessions which kept the integrity of the UK."
The stance of the DUP is particularly important because the party wields influence over some hardline Tory Brexiteers and Mr Johnson is far short of a majority in Parliament.
Speaking on Thursday evening in a joint press conference at the conclusion of Brexit discussions, President of the European Council Donald Tusk and Irish Taoiseach Leo Varadkar both said the door is open for Britain to return to the European Union.
What is the view from Northern Ireland? ITV News Correspondent Peter Smith reports
At the conclusion of his opening statement, Mr Tusk said: "On a more personal note, what I feel today, frankly speaking, is sadness, because in my heart I will always be a remainer, and I hope that our British friends decide to return one day, our door will always be open."
Mr Varadkar echoed the same sentiment:"I have mixed feelings today, I regret the UK is leaving but respect the decision to do so, like an old friend going on a journey without us, and we really hope it works out for them, but I think there will always be a place at the table for them if they ever choose to come back," he said.
President of the European Council Donald Tusk told the press conference: "The reality is that today we have a deal which allows us to avoid chaos and an atmosphere of conflict between the EU 27 and the United Kingdom."
Mr Tusk said the European Council invited the Commission, the European Parliament and the council to ensure that the agreement can enter into force on November 1.
"Now we are waiting for the votes in both parliaments," he said.
Mr Johnson did not rule out suspending the whip from Tories who rebelled on Saturday, or whether he would welcome back the 21 he exiled for previously voting against his will.
"What I will say is I do think there is a very good case for voting for this deal on all sides of the House and we will certainly be taking that vote very seriously," he added when questioned at the press conference.
If Parliament does not vote for the agreement on Saturday, he faces an almighty clash over whether he will request a further Brexit delay from Brussels as he is compelled to under the Benn Act.
The PM insisted the "great new deal" allows the UK to leave the bloc in the nick of time to satisfy his "do or die" commitment to depart by the current October 31 deadline.
Mr Johnson said that the "anti-democratic" backstop contingency measure to prevent a hard border on the island of Ireland had been abolished.
It proved a major sticking point for his predecessor Theresa May whose deal failed to pass the Commons three times.
But the DUP dug in over the prospect of a customs border between Northern Ireland and the rest of the UK, as well as the issues of consent regarding the suspended Stormont Assembly.
Smiles and handshakes in Brussels as deal is agreed. ITV News Europe Editor James Mates looks at why the EU will be glad for the three-year saga to come to a close
Another issue in the PM's proposals was whether EU VAT rates would apply in Northern Ireland.
Mr Johnson needs to get a deal approved before the weekend if he is to avoid a major clash over asking for an extension to the current deadline.
The Benn Act passed by no-deal opponents compels him to ask Brussels for a delay to the end of January, but the PM has repeatedly ruled out taking this course of action.
He remains adamant on his stance, with a senior Government official saying the PM's position is "new deal or no deal, but no delay".
Mr Juncker's no-delay threat could convince some opposition MPs to back what would now appear to be the final offer.
How does Boris Johnson's Brexit deal differ from Theresa May's? Mary Nightingale explains:
However, Labour leader Jeremy Corbyn claimed the deal is "even worse" than Theresa May’s.
He said: "We cannot support this deal and we'll oppose it in Parliament on Saturday."
He added: "This sell out deal won’t bring the country together and should be rejected. The best way to get Brexit sorted is to give the people the final say in a public vote."
Liberal Democrats leader Jo Swinson said her party would support Mr Johnson in Parliament on Saturday, only if they could attach a confirmatory referendum to it.
The SNP has already tabled an amendment to the deal demanding an immediate extension to the October 31 deadline and a general election.
The SNP's Parliamentary leader Ian Blackford said opposition parties need to "quit dithering, back our amendment, and finally act to bring this appalling Tory government down and stop Brexit".
"Boris Johnson's appalling Brexit deal is even worse than Theresa May's," he said.
"It would be devastating for Scotland - dragging us out of the EU, single market and customs union against our will, and singling Scotland out, alone of UK nations, for a raw deal, with our votes and voice ignored.
"The SNP will never vote for this deal, which would inflict lasting harm on jobs, living standards, public services and the economy.
"It is clearer than ever that the best future for Scotland is as an equal, independent European nation and the people of Scotland must be given that choice."