Britain's exit from the European Union hangs on a knife-edge as Prime Minister Boris Johnson scrambles to persuade doubters to rally behind his last-minute European Union divorce deal in an extraordinary vote in parliament.
In one of the most striking flourishes of the three-year Brexit drama, Johnson confounded his opponents on Thursday by clinching a new deal with the EU, even though the bloc had promised it would never reopen a treaty it agreed last year.
Yet Johnson, the face of the Brexit campaign in the 2016 referendum, must on Saturday ratify the deal in the British parliament where he has no majority and opponents are plotting maximum political damage ahead of an imminent election.
The numbers are too close to call: Johnson must garner 318 votes in the 650-seat parliament to get a deal approved. Yet his Northern Irish allies are opposed to a deal and the three main opposition parties have pledged to vote it down.
"We've got a great new deal that takes back control - now parliament should get Brexit done on Saturday," Johnson said before the first Saturday sitting of parliament since the 1982 Argentine invasion of the Falkland Islands.
Johnson won the top job by staking his career on getting Brexit done by the latest deadline of October 31 after his predecessor, Theresa May, was forced to delay the departure date.
Parliament rejected her deal three times, by margins of between 58 and 230 votes.
Downing Street is casting the Saturday vote as a last chance to get Brexit done with lawmakers facing the option of either approving the deal or propelling the United Kingdom to a disorderly no-deal exit that could divide the West, hurt global growth and trigger violence in Northern Ireland.
To win the vote, Johnson must persuade enough Brexit-supporting rebels in both his own Conservative Party and the opposition Labour Party to back his deal.
Concerned about the potential impact of a no-deal departure, Johnson's opponents have already passed a law demanding he delay Brexit unless he gets a withdrawal deal approved by Saturday.
The government has said both that it will comply with this law and that Britain will leave the EU on October 31 whatever happens.
Johnson has not explained how he plans to take these two apparently contradictory steps.
The message from Johnson's advisers is: "New deal or no deal but no delay."
The prime minister was due to hold a cabinet meeting at 3pm GMT on Friday (2am AEDT on Saturday).
As MPs mull one of the United Kingdom's most significant geopolitical moves since World War II, hundreds of thousands of demonstrators are due to march towards parliament demanding another referendum on EU membership.
Parliament will sit from 8.30am GMT (7.30pm AEDT) on Saturday. Johnson will make a statement to MPs, after which there will be a debate and then a vote.
The Northern Irish Democratic Unionist Party (DUP) said it would oppose the deal and lobby a faction of about 28 hardline Brexit supporters in the Conservative Party to do the same.
"We will be encouraging (other MPs to vote against) because we believe it does have an impact on the unity of the United Kingdom, will spark further nationalist sentiment in Scotland and will be detrimental to the economy of Northern Ireland," the DUP's Sammy Wilson said.
Without the DUP's 10 votes, Johnson will need Brexit-supporting Labour Party rebels to support his deal.
Saturday's vote would be "pretty close" but likely just fall short of approval, Labour deputy leader John McDonnell told Sky News.
If the vote is a tie, then the speaker of parliament, John Bercow, would hold the deciding vote. According to vague convention, the speaker would seek to keep the issue open for further discussion.