British Prime Minister Boris Johnson faces a crunch Conservative Party confidence vote later Monday after 54 of his Tory MPs triggered a contest following a string of scandals that have shattered confidence in his leadership.

The beleaguered UK leader has spent months battling to maintain his grip on power after the so-called "Partygate" controversy saw him become the first serving UK prime minister found to have broken the law.

If he loses, he will be forced to step down as Conservative party leader and prime minister.

A scathing internal probe into the scandal said last month that he had presided over a culture of Covid lockdown-breaking parties in Downing Street that ran late into the night and featured a drunken fight among staff.

Johnson, who won a landslide election victory in December 2019 on a vow to "get Brexit done", has steadfastly refused to stand down.

The 57-year-old has said he takes responsibility for the saga and pointed to a reorganisation of the structure inside No 10 while insisting that he must get on with the job.

A Downing Street spokesperson said Johnson "welcomes the opportunity to make his case to MPs" and Monday's vote was "a chance to end months of speculation and allow the government to draw a line and move on".

Threshold triggered

Minutes earlier, Graham Brady, who heads the backbench committee of Conservative MPs which oversees party leadership challenges, confirmed that the threshold of 15 percent of Tory MPs seeking a confidence vote had been exceeded.

"In accordance with the rules, a ballot will be held between 1800 and 2000 (1700 and 1900 GMT) today," he said, noting the votes would be counted immediately afterwards and an announcement then made "at a time to be advised".  

Brady told reporters that Johnson was informed last night -- as four days of national celebrations for Queen Elizabeth II's Platinum Jubilee ended -- that the threshold to trigger a vote had been reached.

"We agreed the timetable for the confidence vote to take place and he shared my view... that vote should happen as soon as could reasonably take place and that would be today," he added.

Brady did not disclose how many letters of no-confidence in Johnson he had received from Conservative MPs, noting some colleagues had post-dated their letters until after the jubilee celebrations.

'Charade'

The 359 Conservative MPs currently sitting in parliament will decide Johnson's fate by secret ballot, and if he wins -- half the votes cast plus one -- the embattled leader cannot be challenged again for a year.

However, the Tory party could change its own internal rules to allow another contest sooner.

If he loses, a leadership contest follows in which he cannot stand.

Speculation has been mounting for months that Johnson would face a challenge to his leadership, as support for him among Conservatives has ebbed away.

Many thought he would face a no-confidence vote earlier this year, as the "Partygate" storm intensified, but the outbreak of the war in Ukraine and Johnson's leading response to it bought him time.

But the end of a police probe into the scandal, which saw dozens of his staff and officials fined for attending illegal gatherings during the pandemic, and the publication of the internal report reignited the controversy.

Tory MP Jesse Norman was the latest to go public with his discontent, posting a withering letter online early Monday that he wrote to Johnson explaining why he had lost faith in him.

Listing a litany of problems with his tenure, Norman wrote: "For you to prolong this charade by remaining in office not only insults the electorate... it makes a decisive change of government at the next election much more likely."

'Stand his corner'

However, cabinet colleagues -- who may stand to succeed Johnson if he is deposed -- jumped to Johnson's defence.

"The Prime Minister has my 100 percent backing in today's vote and I strongly encourage colleagues to support him," Liz Truss said on Twitter.

Health Secretary Sajid Javid told Sky News: "The prime minister will fight and stand his corner with a very, very strong case".

But opinion polls have shown deep public disapproval over "Partygate" and Johnson's handling of the scandal, with large majorities of people saying he knowingly lied about it and that he should resign.

The Tories have suffered several electoral setbacks during his tenure, including losing traditionally safe seats to the Liberal Democrats in by-elections and hundreds of councillors in local elections in early May.

The party is also predicted to lose two more by-elections later this month, in southwest and northern England.