Boris Johnson is clinging onto power by a thread after both Rishi Sunak and Sajid Javid dramatically resigned from government on Tuesday evening.
The prime minister is seen as highly unlikely to resign, but as the pressure on his position grows he could be left with no choice.
Alternatively Tory MPs could soon move to rip up party rules in order to hold another vote of no confidence in Johnson’s leadership.
Johnson survived a no confidence vote last month, and under current rules he is safe for a year. But this can be changed.
How is a no confidence vote triggered?
To mount a leadership challenge, 15% of Conservative MPs have to submit a letter.
Based on the current size of the party in the Commons, this means at least 54 Tory MPs have got out their pens.
The number of letters could be higher than the number known in public, as there is no obligation for MPs to reveal if and when they have handed one in.
The letters are secretly sent to Sir Graham Brady, the veteran Tory MP who leads the body of all backbench Conservative MPs known as the 1922 committee. Only Brady knows how many letters he has received.
There is little doubt the threshold could be reached a second time. In the last vote, on June 6, 148 Tory MPs voted against him and 211 voted in favour.
What happens if he survives the vote?
If some how Johnson managed to survive a second confidence vote, he would remain as prime minister.
What happens if he loses the vote?
If Johnson loses a no confidence vote then a leadership contest is triggered.
He is not allowed to stand for re-election as leader but would likely stay in post until his successor is chosen.
How does the leadership election work?
Conservative Party leadership elections have two parts.
The first stage sees Tory MPs vote in a series of rounds, whittling the list of candidates down to a final two. In the 2019 leadership race this took roughly two weeks.
The second stage sees the final two candidates go to a vote of Tory members. In 2019, when Johnson took on Jeremy Hunt, this took around a month.
Leadership candidates are already seen to be on manoeuvres in Westminster in the expectation of a contest.
The new leader would have until January 2025 before they had to call a general election.