Conservative MPs will vote whether to remove Boris Johnson as leader after at least 54 sent letters of no confidence to Sir Graham Brady, chair of the 1922 committee.
Brady announced on Monday morning that the 54 letter threshold had been reached and that a House of Commons vote will be held this evening between 6-8pm.
A result will be announced "shortly thereafter". If at least half of Conservative MPs – 180 – vote against Johnson, he will be removed from 10 Downing Street and a leadership contest will be triggered.
Downing Street appeared confident that Johnson would command the support of MPs. A spokesperson said the vote this evening was a chance for the government to "draw a line" under weeks of speculation about the prime minister's position.
"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," they said.
"The PM welcomes the opportunity to make his case to MPs and will remind them that when they’re united and focused on the issues that matter to voters there is no more formidable political force".
Brady notified Johnson on Sunday that the threshold had been passed and the pair agreed a timetable for holding the vote tonight, he told reporters outside Westminster.
29 Conservative MPs have publicly called on Johnson to resign amid anger over the partygate scandal and the prime minister's leadership.
The latest to do so was Jesse Norman, the former treasury minister, who in a blistering letter that he published this morning said Johnson's claim that he had been vindicated by Sue Gay's report into lockdown parties held in Downing Street and Whitehall was "grotesque".
Johnson could address a meeting off Conservative MPs later today, before the vote takes places, in an attempt to shore up his support.
Adam Payne is political editor of CSW's sister publication Politics Home, where a version of this story first appeared