Theresa May is fighting to stay on as prime minister after a no confidence vote among Conservative MPs was triggered. How does the vote work and what could happen next?
As leader of the Conservative Party, Theresa May depends on the continued support of her MPs. That has been formally called into question after some of them wrote letters saying they had no confidence in her.
Under the party's rules, at least 15% of Tory MPs must write such a letter to make a leadership challenge possible. That mark - which translates as 48 MPs - has now been passed.
The vote, which is a secret ballot, will be held between 18:00 and 20:00 on Wednesday.
To win, Mrs May needs to secure 159 votes - half of her MPs, plus one. If that happens she will be immune from a further challenge for a year.
But if she loses there will be a leadership contest and she will be barred from standing.
There is a third option - Mrs May wins, but only by a few votes. That might persuade her to think twice about whether she has enough backing in Parliament, and lead her to consider resigning. The result would be a leadership contest.
How a leadership contest would work
The 1922 Committee - the organisation representing all backbench Conservative MPs - decides the procedure and sets the timetable for a leadership contest.
It is likely to follow the following format:
To enter the race, prospective leaders need to be nominated by two fellow Tory MPs.
If more than two wish to stand, MPs whittle down the field to two with a series of ballots where the lowest-ranking person is eliminated after each round.
Then it is time for ordinary party members around the country to vote. The two candidates would be expected to take part in debates to put their cases forward to members before the final voting deadline.
Candidates may withdraw between rounds, and if all but one withdraw, the one remaining candidate will become leader with no need for a vote from members.
Unless there is a general election, the winner of any Tory leadership contest would also become prime minister.