The prime minister faces a decisive vote on Monday evening.

Conservative lawmakers dealt Prime Minister Boris Johnson of Britain a potentially lethal blow to his leadership on Monday when they triggered a no-confidence vote that could force him from power a little more than two years after his landslide election victory.

The move, announced by Graham Brady, who heads a committee of Conservative lawmakers, followed several months of crisis and comes amid claims that Mr. Johnson misled Parliament about lockdown-breaking parties held at Downing Street at the height of a coronavirus lockdown.

Earlier this year Mr. Johnson became the serving first prime minister to be fined by the police for breaking the law for attending a gathering to celebrate his birthday. And last month a report by the senior civil servant, Sue Gray, painted a lurid picture of lawbreaking parties in Downing Street where staff members drank into the early hours, damaged property and on occasion fought with each other.

Facing fierce economic headwinds, with rising inflation and a possible recession looming, Mr. Johnson’s personal popularity has plummeted and there was some booing when he arrived at a service of thanksgiving for Queen Elizabeth’s platinum jubilee last Friday.

“The threshold of 15 percent of the parliamentary party seeking a vote of confidence in the leader of the Conservative Party has been exceeded,” wrote Mr. Brady in a statement adding that the contest would take place between 6 and 8 p.m. Monday evening.

Speaking after the announcement, Mr. Brady said that he had notified Mr. Johnson on Sunday about the situation and that they had agreed that the vote should take place as soon as possible.

Under the rules, Mr. Johnson must now win a simple majority in a vote of his Conservative Party’s lawmakers to remain their leader. That still means that Mr. Johnson has a good chance of survival as his opponents need to muster around 180 votes against the prime minister to topple him, and there is no consensus on who would replace him.

But the vote will be a secret ballot, allowing even those who profess loyalty publicly to withdraw their support if they wish and, if he fails, there will be a contest to replace him as party leader and prime minister.

Despite calls for his resignation and a collapse in his opinion poll ratings, Mr. Johnson had fought hard to try to stem a tidal wave of internal criticism of his behavior and to prevent the confidence vote taking place. For some weeks, his conduct of the war in Ukraine appeared to have won Mr. Johnson a reprieve.

Despite those efforts, Mr. Brady confirmed that the threshold of 54 letters from Conservative lawmakers that was needed to secure a no-confidence vote had been passed. On Monday the health secretary, Sajid Javid, told the BBC that Mr. Johnson would fight to stay on and appealed to Conservative lawmakers to unite around the prime minister and vote in favor of him.

The last time such a no-confidence vote took place, in 2018, it followed quickly after a similar announcement from Mr. Brady. Theresa May, then the prime minister, survived the contest but was still forced to resign several months later.