LONDON (AP) – British lawmakers are holding their first weekend sitting in almost four decades to vote on whether to approve Prime Minister Boris Johnson’s Brexit deal with the European Union and finally let the UK leave the bloc at the end of this month.

At least, that’s the government’s plan. Some lawmakers have other ideas, and may yet delay the final decision — more than three years after British voters opted to leave the 28-nation bloc.

Since striking a deal with the 27 other EU nations on Thursday, Johnson has been imploring and arm-twisting both Conservative and opposition lawmakers as he tries to win majority support for his deal.

Johnson’s Conservative Party holds only 288 seats in the 650-seat House of Commons, so he will have to rely on support from other parties and independent lawmakers to get over the line.

The result looks set to be close, although Johnson has had some success winning over both hard-core Conservative Brexiteers and a handful of opposition Labour lawmakers who represent pro-Brexit parts of the country.

Johnson hopes for success in getting a fractious Parliament to back the deal after his predecessor, Theresa May, failed three times to get lawmakers behind her plan.

He said in The Sun newspaper on Saturday that a vote for the plan would bring a “painful chapter in our history” to an end.

As lawmakers gathered inside Parliament — their first Saturday sitting since the 1982 Falklands War — tens of thousands of anti-Brexit demonstrators were expected to march on the building, calling for a new referendum on whether to leave the EU or remain.

Saturday’s vote could still be derailed.

Many lawmakers want to rule out the possibility that Britain could leave the bloc without a deal on the Oct. 31 deadline. A proposed amendment to Saturday’s vote would withhold approval of the deal until all the necessary legislation to implement it has passed.

One of the lawmakers behind the measure, Oliver Letwin, said it would prevent the UK from leaving at the end of the month “by mistake if something goes wrong during the passage of the implementing legislation.”

If that amendment is chosen for a vote by House of Commons Speaker John Bercow and passes, Johnson will have to ask the EU for a delay to Britain’s departure date. Last month Parliament passed a law compelling the government to do that if no deal is approved by Saturday.

It would also give another chance to scrutinize — and possibly change — the departure terms while the legislation is passing through Parliament.

Johnson’s hopes of getting the deal through Parliament were dealt a blow when his Northern Ireland ally, the Democratic Unionist Party, said it would not back him. The party says Johnson’s Brexit package — which carves out special status for Northern Ireland to keep an open border with EU member Ireland — is bad for the region and its bonds with the rest of the UK.

“We will not be supporting the government, we will be voting against,” said the party’s deputy leader, Nigel Dodds. “Because it isn’t Brexit for the whole of the United Kingdom.”

To make up for the votes of 10 DUP lawmakers, Johnson has tried to persuade members of the left-of-centre Labour Party to support the deal. Late Friday the government promised to bolster protections for the environment and workers’ rights, to allay Labour fears that the Conservative government plans to slash those protections after Brexit.

Senior figures in the party were unimpressed.

“This empty gesture is not worth the paper it’s written on,” said Labour employment spokeswoman Laura Pidcock.

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Mike Corder contributed to this story.