The PM retained strong support from the Tory backbenches throughout his handling of the Covid-19 pandemic but has struggled to keep his party's backing after being accused of breaking his own lockdown rules.
Despite being fined by police for breaking the law, the seemingly Teflon prime minister had managed to stave off any rebellion until Sue Gray published her long-awaited report into the Partygate scandal.
The report was highly damning, attacking a "failure of leadership" in Number 10, but to the surprise of many, the floodgates did not open to a torrent of no confidence letters.
Instead, there's been a steady drip-drip of Tories submitting letters of no confidence to the 1922 Committee after taking time to consider Ms Gray's report and the reaction to it from their constituents.
On the Monday after the long Jubilee weekend, Jesse Norman became the latest Tory MP to announce that he had submitted a letter to Sir Graham.
In a letter to the prime minister 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”.The former minister said Mr Johnson’s current policy priorities were “deeply questionable” and that there were no circumstances in which he could serve in a government led by him.Carlisle MP John Stevenson added his name to the growing list on May 31, saying he'd been left with "no option" but to submit a letter in a bid to force a confidence vote because the PM was "unwilling" to submit himself to one.
In a statement on Monday, Sir Graham, said: “The threshold of 15% of the parliamentary party seeking a vote of confidence in the leader of the Conservative Party has been exceeded.
“In accordance with the rules, a ballot will be held between 1800 and 2000 TODAY MONDAY 6th JUNE — details to be confirmed.
“The votes will be counted immediately afterwards. An announcement will be made at a time to be advised. Arrangements for the announcement will be released later today.”
He said some colleagues had post-dated their letters until after the Platinum Jubilee celebrations.
Which Tories have submitted letters of no confidence and do they stand a chance of ousting him?
While the threshold of letters has been hit, just 18 Tory MPs have penned theirs publicly.
1. Aaron Bell 2. Andrew Bridgen3. Will Wragg 4. Caroline Nokes 5. Gary Streeter6. Steve Brine7. Elliot Colburn8. Stephen Hammond 9. Mark Harper10. Alicia Kearns11. Tobias Ellwood12. Sir Roger Gale 13. Nick Gibb14. Anthony Mangnall15. Bob Neill16. Peter Aldous17. John Stevenson18. Jesse Norman
There is a long list of Tory MPs who have publicly urged the prime minister to resign however they have not confirmed whether they've submitted letters.
Tory MPs who have told Boris Johnson to quit and could have submitted letters:
1. Andrea Leadsom2. David Davis3. Steve Baker4. Neil Hudson5. David Simmonds6. John Baron7. Angela Richardson8. Karen Bradley9. Tim Loughton10. Andrew Mitchell11. Nigel Mills12. Anne Marie Morris13. Julian Sturdy14. Craig Whittaker 15. Jeremy Wright16. Robert Syms
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What is the process for backbench Tories to remove their leader?
Tory MPs are able to force a vote of no confidence in their leader if they won't resign.
To do so requires 15% of the parliamentary party to submit letters of no confidence to the 1922 Committee, which is effectively a HR department for backbenchers.
It takes 54 letters of no confidence to trigger a secret ballot, with a simple majority required for either side to win.
If more than 50% of Tory MPs vote to remove him, he will lose his role of party leader and be barred from competing in the forthcoming leadership election.
If the leader wins over half the votes, then they remain party leader and are given a year's immunity from any further confidence votes.
If a party leader loses a confidence vote then they will be banned from standing in the forthcoming contest and MPs from across the party can be nominated as potential replacements.
The 1922 Committee will determine how many nominations an MP will require to appear on the ballot.
If more than two qualify then MPs will vote on their preference, leaving two final candidates who must then appeal to party members for votes before being elected leader.
How does the voting work?
MPs will vote in a secret ballot on Monday between 6pm and 8pm in a committee room in the Palace of Westminster. If an MP is not in Westminster, they can nominate a colleague to vote on their behalf as a proxy.
MPs will be asked whether they continue to have confidence in the prime minister - and will mark their ballot paper with an x by 'yes or no'.
Photographs are prohibited in the room, and if an MP is caught taking pictures, their vote will be void.
What are their chances of ousting Boris Johnson?
The odds are stacked in the prime minister's favour despite dissenting backbenchers forcing a ballot thanks to the huge majority he won in 2019 by bulldozing Labour's red wall. Many of 2019's intake are aware they owe their seat in the Commons to Mr Johnson's popularity.
While at least 54 letters have been submitted calling for a vote, it would take 180 MPs to agree he should be removed in order to elect a replacement.
Given that just 32 Tory MPs have publicly called on him to quit, even after he was fined by the police, it appears unlikely another 148 would decide they want rid of him as soon as a ballot is triggered.