If the amendment passes, it also will force Johnson to seek a delay from the EU to Britain's departure, due to take place in less than two weeks on October 31.
The prime minister signaled that he would do that under duress. He is compelled by law to ask for the extension, but he said "it cannot change my judgment that further delay is pointless, expensive and deeply corrosive of public trust."
Since striking a deal with the EU 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.
Many lawmakers want to rule out the possibility that Britain could crash out of the bloc without a deal on the October 31 deadline — a prospect economists say would disrupt trade and plunge the economy into recession. The amendment to be voted on Saturday 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 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-center 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.
- AP
