Sir Graham Brady, chairman of the backbench 1922 Committee, confirmed this morning that he has now received the 54 letters from party colleagues needed to trigger a vote.
The secret ballot, which will take place at Westminster at between 6pm and 8pm, raises the possibility that Mr Johnson’s tenure could come to a crashing end less than three years after he won a stunning 80-strong Commons majority.
It comes after a steady stream of Tory MPs called publicly for the Prime Minister to stand down in the wake of Sue Gray’s report into illegal Covid parties in No 10 and Whitehall.
In order to oust the Prime Minister however the rebels will need 180 MPs, and allies of Mr Johnson – including members of the Cabinet – have made clear he is determined to fight to stay on.
Here are the key questions about the upcoming vote.
What is the mechanism for removing the Tory leader?
Conservative Party rules allow MPs to force a vote of no confidence in their leader, with much of the responsibility resting in the hands of the chairman of the backbench 1922 Committee, Sir Graham Brady.
A total of 54 letters of no confidence from Tory MPs, representing 15 per cent of the 360-strong parliamentary party, are needed to trigger a vote.
Sir Graham has received a steady stream of letters in recent days as Tories spent time in their constituencies consulting local parties about their position.
He announced this morning that the threshold had been reached – after waiting for the Platinum Jubilee celebrations to conclude.
At least 50 per cent of Tory MPs must vote ‘no confidence’ for the Prime Minister to lose.
Most predictions suggest Boris Johnson is likely to win today’s no confidence vote, but he could still emerge wounded
How many Tory MPs have sent letters so far?
Letters are confidential unless the MP sending it makes it public.
That means only Sir Graham knows how many letters there are at any one time – and he is famously secretive about the numbers.
Almost every day last week Tory MPs declared they had submitted letters of no confidence in their leader.
More than 30 Tory MPs have publicly urged the PM to resign amid the fallout from Partygate, but they have not all confirmed whether they have submitted letters.
Jesse Norman – a long-term supporter of Mr Johnson – today became the latest Tory MP to announce that he had submitted a letter to Sir Graham.
In a letter to the PM posted on social media, Mr Norman, the MP for Hereford and South Herefordshire, said Mr Johnson had presided over ‘a culture of casual law-breaking’ in No 10 and that his claim to be vindicated by the Sue Gray report was ‘grotesque’.
Earlier this year Mr Johnson became the first serving UK Prime Minister found to have broken the law.
He refused to stand down after receiving a single fine for attending a lockdown-breaching event in Downing Street in 2020 during the coronavirus pandemic.
But support for him among Conservatives has ebbed away in recent weeks following the publication of an internal inquiry which found he presided over a culture of such parties that ran late into the night and even featured a drunken fight among staff.
A host of Tory MPs have come forward to say they do not believe the party can win the next general election under Mr Johnson’s leadership.
Opinion polls have shown deep public disapproval over the scandal, with large majorities of people saying he knowingly lied about ‘Partygate’ 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.
What happens between now and the vote this evening?
Between now and the vote between 6pm and 8pm this evening, Tory MPs will be subjected to a frantic wave of lobbying by the Prime Minister and his allies.
MPs will also be collared for their voting intentions in order to target lobbying efforts at potential rebels.
Cabinet ministers have immediately rallied round Mr Johnson, so will – outwardly at least – be joining attempts to push the vote in his favour.
Opponents of the PM are likely to be similarly active.
How with the physical voting process work?
A ballot box that is kept in the office of one of the 1922 Committee’s executive members will be used for the vote.
It is likely the box will be placed in the same room where MPs gather for meetings of the committee each week, at the centre of a long corridor on the first floor of the Palace of Westminster, overlooking the Thames.
Tory MPs will vote between 6pm and 8pm by secret ballot and, just like at a general election, photography will be banned.
This is intended to prevent the PM’s allies from demanding photographic evidence that an MP has voted in his favour.
The idea is for MPs to be allowed to vote according to their conscience. Any MP who is away from Westminster can nominate a proxy to vote on their behalf.
Sir Graham Brady oversees the process as chair of the backbench 1922 Committee
When will the result be announced?
The process is a simple yes-or-no vote is held, with the leader technically only needing to win support from a simple majority of MPs.
Sir Graham said arrangements for when the announcement would take place will be released ‘later today’.
Given results will be counted straight away and there are not a particularly large number of ballot papers to go through, it is likely to take place very quickly.
It is likely Sir Graham will do as he did in 2018, and assemble MPs and journalists in the same committee room where voting took place earlier in the day.
He will then announce that the parliamentary party does or does not have confidence in Mr Johnson, before revealing the number of votes cast on each side.
Sir Graham said he hoped there would be a ‘clear result’.
What happens if he loses?
The leader is sacked if they do not win a majority of votes from MPs, and a leadership contest begins in which they cannot stand.
However, they typically stay on as Prime Minister until a replacement is elected.
In a leadership contest MPs whittle down the field of contenders to two before party members have the final say.
How does a Tory leadership contest work?
If Mr Johnson is voted out or forced to resign, a leadership contest to replace him as the head of the Tory Party would take place – although he is likely to remain in post as Prime Minister until a successor is in place.
The contest takes place in two stages.
In the first stage, Conservative MPs put themselves forward as candidates.
All Tory MPs then vote in a series of rounds to reduce the number of candidates until only two remain.
The second stage of the contest sees the two remaining candidates put to a vote of Conservative Party members.
And if he survives?
If Mr Johnson survives, under Conservative Party rules no further confidence vote can be held by the 1922 Committee for another year.
However, Sir Graham today acknowledged that Mr Johnson will not necessarily be safe even if he survives tonight’s vote.
Although the Conservative committee’s rules say there could not be another confidence vote for 12 months, Sir Graham said those procedures could be changed.
Theresa May – pictured with her husband Philip at the Platinum Jubilee Thanksgiving service at St Paul’s – also faced a no confidence vote
‘Technically it’s possible for rules to be changed but the rule at present is there would be a period of grace,’ he told reporters.
But even if Mr Johnson survives the vote this evening, his leadership could be fatally undermined if a significant number of MPs vote against him.
Could he resign anyway if he only wins by a narrow margin?
Technically yes, but the signals coming out of Number 10 suggest he is determined to fight on.
‘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,’ a Downing Street spokesman 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.’
His reputation as a political survivor who has resisted a raft of other calls to resign in the past also suggests he would have to be forced out.
What if he decided to quit immediately?
The cabinet would agree on a temporary successor while a leadership contest took place.
This is likely to be Mr Johnson’s deputy, Dominic Raab.
Who could replace Mr Johnson?
One major problem for the rebels is the lack of an obvious replacement for Mr Johnson.
Rishi Sunak, previously regarded as the favourite, was also fined over Partygate and there is no other front runner.
Former Cabinet minister Jeremy Hunt has been touted as a contender, while Defence Secretary Ben Wallace is riding high with party grass roots. Tom Tugendhat is the only MP to have openly declared he wants to be PM.
When was the mechanism to replace the leader last used?
The last confidence vote was held in December 2018 when, after months of speculation, enough MPs decided they no longer had faith in Theresa May’s leadership.
She won the contest 200 votes to 117 – making her immune to another challenge for a year.
But she was forced to announce her resignation the following spring amid another insurrection over Brexit, paving the way for Mr Johnson to take the helm.