Boris Johnson has ramped up his criticism of Theresa May’s Brexit plan – a move many in Westminster see as a bid to boost his own leadership credentials.
The former foreign secretary said she had “gone into battle with the white flag fluttering over our leading tank”, trying to trash her record on negotiations with the EU.
The prime minister has previously insisted she will lead the Conservatives into the next general election, but a number of Tory MPs are said to be ready to back any moves to topple her leadership.
But what do they need to do to force a leadership challenge?
:: The magic numbers – 1922 and 48
Mrs May’s fate hangs in the balance of the 1922 committee – the group of all backbencher Conservative MPs.
To trigger a vote of no confidence, its chair has to receive letters from 15% of them – 48 at present – formally calling on the leader to stand down.
The process is completely blind, though, so only the current chair Graham Brady knows how many letters have been submitted and by whom.
Several MPs publicly announced they had put in letters as part of a bid to unseat Mrs May in July, following the resignations of Boris Johnson and former Brexit secretary David Davis.
They remain on file, unless withdrawn.
:: What happens next?
A date for the first round of the leadership election would be chosen by Mr Brady, in consultation with the prime minister.
Contenders to replace her will be nominated, but must get the backing of 15% of Tory MPs.
Depending on how many candidates there are, rounds of elections will be held, with the MPs with the fewest votes dropping out of the race one by one.
That all changes when there are two candidates left – Conservative Party members will have the final say in an open election.
Mrs May managed to win without going through this final phase, because her opponent, now-Commons leader Andrea Leadsom, dropped out for suggesting being a mother made her better qualified.
If there is a contest, the winner will be announced by Mr Brady.
If all of Parliament wants to get rid of the prime minister, it has to table a vote of no confidence.
All 650 MPs would get the chance to vote formally on whether Mrs May should stay leader or resign, with the expectation a general election would follow.