Theresa May is fighting to hold on to her job as Conservative MPs prepare to cast their ballots in a vote of no confidence this evening.
But how will the vote work and what might happen next?
Here we aim to answer all your questions.
Why is it happening?
May has been under pressure since the summer, when Tory MPs began submitting letters of no confidence in the Prime Minister as Brexit negotiations dragged on.
Last month May unveiled her deal to parliament, prompting Eurosceptic Tory Jacob Rees-Mogg to submit a letter of his own as MPs on all sides voiced anger over the nature of the deal.
Under party rules, 15 per cent of Tory MPs needed to write to the party's 1922 Committee to trigger a leadership challenge. That threshold of 48 MPs has now been passed, but only the committee’s chairman, Sir Graham Brady, knows exactly how many wrote to declare their lack of confidence in the Prime Minister.
When will the vote occur?
The vote will be held in secret between 6pm and 8pm in the House of Commons this evening.
When will we know the result?
Sir Graham has said the votes will be counted immediately after the ballot and that we should know the result by around 9pm.
What are the possible outcomes?
May will need a majority of the total votes if she is to remain party leader. Provided all Conservative MPs cast their ballots, or send proxies in their places, this means she needs a total of 158 votes.
If May wins the vote, she can stay on as Prime Minister and cannot be challenged as Conservative leader for another year.
But if the margin of victory is small, the authority of her premiership – and her Brexit deal – will be significantly weakened.
If May loses, a leadership contest will be held to to decide her replacement.
Candidates must be nominated by two of their fellow Tory MPs. If only one candidate is elected they will automatically take over as prime minister and leader of the Conservative party.
If more than two candidates are nominated, further secret votes will be held among Conservative lawmakers to whittle down the field.
Following each ballot, the candidate with the fewest votes is eliminated. The process is then repeated every Tuesday and Thursday until only two remain.
The leader of the Conservative party will then be elected by party members across the country through a postal ballot. Participants need to have been a member of the party for more than three months in order to vote.
Dominic Raab, former foreign secretary Boris Johnson, and cabinet minister Penny Mordaunt have all been touted as possible Eurosceptic candidates for a leadership contest. Remain-voting Sajid Javid and Amber Rudd are also seen as contenders. There is no clear frontrunner, however.
Theresa May became head of government through this same process after David Cameron stepped down as Prime Minister and Conservative leader in 2016. The field was narrowed down to May and then-junior minister Andrea Leadsom, but Leadsom pulled out before the final vote.