What you need to know:
- Parliament was holding its first Saturday sitting since the 1982 Falklands War to discuss the terms of a last-ditch divorce agreement Johnson struck with European Union leaders Thursday.
- The prime minister needs a clear vote in favour of his deal on Saturday to avoid triggering a law requiring him to ask the EU to delay Brexit for the third time.
- Any extension would depend on all 27 EU leaders saying yes. However, Britain could still leave the EU on October 31.
Prime Minister Boris Johnson fought attempts Saturday to potentially further delay Britain's protracted departure from the European Union, as MPs debated his divorce deal less than two weeks before the Brexit deadline.
Parliament was holding its first Saturday sitting since the 1982 Falklands War to discuss the terms of a last-ditch divorce agreement Johnson struck with European Union leaders Thursday.
Opposition parties and Johnson's own Northern Irish allies have rejected the text, forcing frantic government attempts to try to win the support of wavering MPs.
The prime minister needs a clear vote in favour of his deal on Saturday to avoid triggering a law requiring him to ask the EU to delay Brexit for the third time.
The Conservative leader says it is time to bring some closure to a tortuous process sparked by the 2016 referendum vote for Brexit, which has plunged Britain into political turmoil and divided the nation.
But support is growing for an amendment to withhold MPs' approval unless and until legislation required to ratify the treaty has passed.
Its author, Oliver Letwin, a former Conservative minister, fears the ratification process may not be complete by October 31 -- risking an accidental "no deal" Brexit.
Backed by the main opposition, the amendment would have the effect of forcing Johnson to ask to delay -- something he has said he would "rather be dead in a ditch" than do.
Any extension would depend on all 27 EU leaders saying yes. However, Britain could still leave the EU on October 31.
The government intends to bring forward the ratifying law on Monday, and it could still pass parliament by the end of the month.
Addressing the lower House of Commons ahead of the vote, Johnson said further delay would be "pointless, expensive and deeply corrosive to public trust".
He said his deal would be a "new way forward and a new and better deal both for Britain and our friends in the EU".
Securing a deal was a personal victory for Johnson, a figurehead in the 2016 Leave campaign who has vowed to deliver Brexit on October 31 come what may.
But parliament -- like the frustrated public -- is still bitterly divided over how and even if Britain should end almost half a century of integration with its closest neighbours.
The debate coincided with a mass demonstration to parliament demanding a second referendum, with an option to reverse Brexit.
Inside parliament, many hardline eurosceptic MPs in Johnson's Conservative party offered their support.
MP Mark Francois added: "If the House of Commons votes for this today, within a fortnight we'll be living in a free country."
Johnson took office in July after his predecessor Theresa May failed three times to get her own divorce deal through parliament.
He insists that Brexit must happen this month to end the uncertainty that has weighed on the economy and dominated political and public debate.
However, the vote rests on a knife-edge.
Northern Ireland's Democratic Unionist Party, which supports Johnson's minority government, said it will vote against the agreement because of its arrangements for the British province.
The main opposition Labour party says the deal lacks sufficient protections for environmental and workers' rights, and puts Britain on a path to a loose trading relationship with the EU after Brexit that could hit the economy.
"We will not back this sell-out deal," leftist leader Jeremy Corbyn said.
He added: "Voting for a deal today won't end Brexit. It won't deliver certainty and the people should have the final say."
Not far from parliament, thousands of protesters gathered for a demonstration demanding a new referendum, with an option to reverse Brexit or at least have a say on Johnson's deal.
"The first referendum was jumping on a train without a destination. Now that we have a destination, we need to have a second referendum," said Douglas Hill, 35, on the march with his Estonian wife.
An online Survation poll of 1,025 adults on Thursday and Friday for the Daily Mail found that 50 percent said MPs should vote for the deal, while 38 percent said not.
EU leaders in Brussels this week urged lawmakers to back the deal to allow both sides to move on to discussing their future relationship.
The deal covers Britain's financial settlement, protects the rights of EU citizens and sets out a post-Brexit transition period potentially until 2022 to allow both sides to agree new trade terms.