Associated Press: LONDON, Jill Lawless — British Prime Minister Boris Johnson will face a no-confidence vote Monday that could oust him from power, as discontent with his rule finally threatens to topple a politician who has often seemed invincible despite many scandals.

If Johnson loses the vote among the 359 Conservative lawmakers, the party will choose a new leader, who will also become prime minister. If he wins, as seems more likely, he can’t face another challenge for a year under current party rules — though a narrow victory would leave him a hobbled leader whose days are likely numbered.

FILE – Britain’s Prime Minister Boris Johnson and his wife Carrie, are seated during the Platinum Jubilee Pageant, in London, Sunday June 5, 2022, on the last of four days of celebrations to mark the Platinum Jubilee. Britain’s governing Conservatives will hold a no-confidence vote in Prime Minister Boris Johnson on Monday, June 6, 2022 that could oust him as Britain’s leader – Chris Jackson/Pool Photo via AP

Regardless of the outcome, the fact that enough lawmakers are demanding such a vote represents a watershed moment for Johnson. It is also a sign of deep Conservative divisions, less than three years after Johnson led the party to its biggest election victory in decades.

Johnson’s renown as an election-winner has previously insulated him from the fallout of a growing number of scandals. But recently he has struggled to turn the page on revelations that he and his staff repeatedly held boozy parties that flouted the COVID-19 restrictions they imposed on Britain in 2020 and 2021.

Conservative Party official Graham Brady announced Monday that he had received letters calling for a no-confidence vote from at least 54 Tory legislators, enough to trigger the measure under party rules.

“The threshold of 15 percent (of Conservatives in the House of Commons) has been passed,” Brady said. He said the vote would take place in person in the Commons on Monday evening, with the result announced soon after.

Johnson’s Downing Street office said the prime minister welcomed the vote.

FILE – An anti-Conservative Party protester holds a placard with an image of British Prime Minister Boris Johnson including the words “Now Partygate” backdropped by the Houses of Parliament, in London, Wednesday, Dec 8, 2021 – AP Photo/Matt Dunham

“Tonight is a chance to end months of speculation and allow the government to draw a line and move on, delivering on the people’s priorities,” it said.

Johnson’s supporters think he will win the backing of more than 180 lawmakers and defeat the challenge. But he could still emerge severely weakened. The last prime minister to survive a no-confidence vote was Theresa May in 2018. She never regained her authority and resigned within months, sparking a leadership contest that was won by Johnson.

His selection in July 2019 capped a rollercoaster journey to the top. He had held major offices, including London mayor and UK foreign secretary, but also spent periods on the political sidelines after self-inflicted gaffes. He kept bouncing back, showing an uncommon ability to shrug off scandal and connect with voters that, for many Conservatives, overshadowed doubts about his ethics or judgment.

But qualms have been growing, and came to a head after an investigator’s report late last month that slammed a culture of rule-breaking inside the prime minister’s Downing Street office in a scandal known as “partygate.”

FILE – Anti-Boris Johnson and anti-Brexit protesters hold placards by Parliament Square, in London, Wednesday, Jan 26, 2022. Britain’s governing Conservatives will hold a no-confidence vote in Prime Minister Boris Johnson on Monday, June 6, 2022 that could oust him as Britain’s leader – AP Photo/Matt Dunham

Civil service investigator Sue Gray described alcohol-fueled bashes held by Downing Street staff members at a time when pandemic restrictions prevented UK residents from socializing or even visiting dying relatives.

Gray said the “senior leadership team” must bear responsibility for “failures of leadership and judgment.”

Johnson also was fined 50 pounds ($63) by police for attending one party, making him the first prime minister sanctioned for breaking the law while in office.

The prime minister said he was “humbled” and took “full responsibility” — but insisted he would not resign. He urged Britons to “move on” and focus on the battered economy and the war in Ukraine.

But a growing number of Conservatives feel that Johnson is now a liability.

FILE – British Prime Minister Boris Johnson leaves 10 Downing Street to attend the weekly Prime Minister’s Questions at the Houses of Parliament, in London, Wednesday, May 25, 2022 – AP Photo/Matt Dunham

Lawmaker Jesse Norman, a longtime Johnson supporter, said the prime minister had “presided over a culture of casual law-breaking” and had left the government “adrift and distracted.”

“I am afraid I can see no circumstances in which I could serve in a government led by you,” Norman wrote in a letter published on social media.

If Johnson is ousted it would spark a Conservative leadership contest, in which several prominent government ministers are likely to run.

Conservative lawmaker Roger Gale, a Johnson critic, said “we have some very good alternatives to the prime minister so we’re not short of choice.”

“Any single one of those people in my view would make a better prime minister than the one that we’ve got at the moment,” he told the BBC.

FILE – British Prime Minister Boris Johnson and his wife Carrie Symonds arrive for a service of thanksgiving for the reign of Queen Elizabeth II at St Paul’s Cathedral in London, Friday, June 3, 2022 on the second of four days of celebrations to mark the Platinum Jubilee – AP Photo/Matt Dunham, Pool

Discontent seems to have come to a head over a parliamentary break that coincided with celebrations of Queen Elizabeth II’s Platinum Jubilee. For many, the four-day long weekend was a chance to relax — but there was no respite for Johnson, who was booed by some onlookers as he arrived for a service in the queen’s honor at St. Paul’s Cathedral on Friday.

Senior ministers — some of them likely candidates in a future leadership contest — tweeted messages of support for Johnson.

“The Prime Minister has my 100 percent backing in today’s vote and I strongly encourage colleagues to support him,” said Foreign Secretary Liz Truss, one of the favorites to succeed Johnson.

Cabinet minister Steve Barclay, a Johnson ally, said toppling the leader now would be “indefensible.”

“The problems we face aren’t easy to solve,” he wrote on the Conservative Home website. “But under Boris Johnson’s leadership, our plan for jobs shows how we are navigating through these global challenges.”

“To disrupt that progress now would be inexcusable to many who lent their vote to us for the first time at the last general election, and who want to see our Prime Minister deliver the changes promised for their communities,” he added.

Top Feature Photo: FILE – British Prime Minister Boris Johnson listens to the Amir of Qatar Sheikh Tamim bin Hamad Al Thani speak at the start of their meeting inside 10 Downing Street, in London, Tuesday, May 24, 2022. Britain’s governing Conservatives will hold a no-confidence vote in Prime Minister Boris Johnson on Monday, June 6, 2022 that could oust him as Britain’s leader – AP Photo/Matt Dunham, Pool