Britain’s prime minister, beset by a scandal over lockdown parties, faces a decisive vote.

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

The move 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 pandemic.

Voting has ended for the no confidence motion and party officials are expected to announce at 9 p.m. (4 p.m. Eastern) whether Mr. Johnson has retained leadership of the party, and therefore his job as Britain’s prime minister.

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

Facing fierce economic headwinds including rising inflation, 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.

Graham Brady, who heads a committee of Conservative lawmakers, said in a statement that more than 15 percent of Conservative members of Parliament had sought a vote of confidence, surpassing the threshold necessary to force a ballot.

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

The vote was secret, allowing even those who profess loyalty publicly to quietly withdraw their support if they wish. 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 stem internal criticism of his behavior and to prevent the confidence vote taking place. For some weeks, his hard-line approach to Russia for its invasion of Ukraine appeared to have won him a reprieve.

In a speech to his fellow Conservative lawmakers, Mr. Johnson argued that the best was yet to come if they continued to support him. “I will lead you to victory again and the winners will be the people of this country,” Mr. Johnson said, according to excerpts from the text released by a party official.

The last time a similar no-confidence vote took place was in 2018. Theresa May, then the prime minister, survived the contest but was still forced to resign several months later.

Writing on Twitter, the foreign secretary, Liz Truss — who is seen as a potential successor to Mr. Johnson — expressed her support for him.

One former minister, Jesse Norman, laid out a case against Mr. Johnson in a letter published Monday, saying, among other things, that the prime minister had “presided over a culture of casual lawbreaking at 10 Downing Street in relation to Covid.”