LONDON — Prime Minister Boris Johnson tried to quell a rebellion among lawmakers from his Conservative Party on Monday as Parliament prepared to vote on Brexit legislation that the government acknowledges would violate international law.
The vote, likely to happen late on Monday evening after a critical debate in the House of Commons, is only the first step for the legislation, which would nullify parts of a landmark agreement that Mr. Johnson struck with the European Union last fall, paving the way for Britain to leave the bloc after 44 years.
While Mr. Johnson, with an 80-seat majority, is all but certain to prevail in the vote, it sets in motion a politically perilous period for him, both with his own party and with the European Union, which has warned that the legislation would torpedo talks for a post-Brexit trade deal.
With further legislative hurdles to come, analysts said that the danger was less a quick defeat than a gradual leaching away of support that could leave Mr. Johnson weakened at a time when his government is battling a resurgence of the coronavirus and the effects of a lockdown-ravaged economy.
“If you put together concerns about the government’s handling of the virus, with concerns about the economy, and concerns about the effects on individual liberties of the lockdown, Boris Johnson is making a lot of people unhappy,” said Tim Bale, professor of politics at Queen Mary University of London.
Britain also faces other international repercussions, not least with the United States, where congressional Democrats have warned Mr. Johnson that his move could scuttle a trans-Atlantic trade deal because it undermines the Good Friday Agreement, which ended decades of sectarian violence in Northern Ireland.
Mr. Johnson’s aggressive move to rewrite provisions of the treaty relating to Northern Ireland has drawn a torrent of criticism from prominent Conservative figures, including former Prime Minister David Cameron, who said on Monday that he had “misgivings” about the government’s proposal.
“Passing an act of Parliament and then going on to break an international treaty obligation is the very, very last thing you should contemplate,” Mr. Cameron told reporters, becoming the latest of five former prime ministers — three of them Conservatives — to speak out against Mr. Johnson’s plan.
The government also lost the support of a former attorney general, Geoffrey Cox, who served in Mr. Johnson’s last cabinet and who said he would vote against the law because it would do “unconscionable” harm to Britain’s global standing. His defection was noteworthy because Mr. Cox favored Brexit and was the government’s top legal adviser when Mr. Johnson negotiated the withdrawal agreement.
Despite the boldface names lining up to oppose the law, there was no sign that Mr. Johnson planned to back down. Threatening to rip up an agreement with the European Union plays well with the hard-line Brexiteers in his party. And there are still three-and-a-half months before the Dec. 31 deadline for a trade deal with Brussels, which means Mr. Johnson could always compromise later.
Mr. Johnson pursued a similar strategy of brinkmanship this time last year, threatening to leave the European Union without a withdrawal agreement, which prompted a similar, if more widespread mutiny, in his party.
The prime minister expelled 21 rebels for defying the government, including Conservative grandees such as Kenneth Clarke, a former chancellor of the Exchequer; and Nicholas Soames, a grandson of Winston Churchill. Government officials said that Downing Street was considering a similar punishment this time.
Mr. Johnson kept threatening a no-deal Brexit until October, when he met with Leo Varadkar, then the prime minister of Ireland, and suddenly cut a deal on the treatment of Northern Ireland that opened the door to a broader agreement with Brussels. Mr. Johnson presented the compromise as a striking victory.
It is that deal that Mr. Johnson now proposes to renege on, if Britain is unable to agree to long-term trade arrangements with the European Union. The government says that the legislation is intended to provide a “safety net” for businesses in Northern Ireland, which send and receive goods from the rest of the United Kingdom.
Few analysts expected more than a handful of Conservative lawmakers to vote against the government on Monday, since the vote was on the direct question of whether to advance the legislation, known as the Internal Market Bill.
But more may vote for an amendment — to be introduced next week by a Conservative member, Bob Neill — that would block the government from using the provisions of the bill that would break the law by nullifying the so-called Northern Ireland protocol, without the approval of Parliament.
Even that rebellion seems likely to fall short, however, given the size of Mr. Johnson’s majority and the reluctance of Conservative lawmakers to fall out with a prime minister who delivered a landslide victory less than a year ago.
If the legislation gets through the House of Commons, it goes to the House of Lords, where Michael Howard, who sits in that chamber and who is a former head of the Conservative Party, predicted it would run into a buzz saw of opposition. Still, the Lords rarely thwart legislation that has passed the lower house.
For all the fireworks, some analysts still said that they believed Mr. Johnson would ultimately come to terms with Brussels. The prime minister’s threats to break off talks, breach international law or accept a no-deal Brexit are all intended, they said, to prepare the ground for a compromise.
“This is a signal to his Brexit ultras that he is playing hardball with the European Union, which will make it easier when he eventually has to make concessions to cut a deal,” Professor Bale said.
The risk for Mr. Johnson, analysts said, is that he could miscalculate the reaction from the European Union, which has so far been measured, or that his government no longer has the bandwidth to conclude a trade deal — a scenario that is not implausible given its growing urgency about a second wave of the virus.
“It is perfectly possible to slide into war,” Professor Bale said.