Theresa May faces a leadership challenge from Tory MPs after the threshold of 48 letters of no confidence was reached.
But what should happen next, how would a leadership contest work, and what does all this mean for Brexit?
What happens in the no confidence vote?
Sir Graham Brady, chairman of the Conservative backbench 1922 Committee, said the threshold had been exceeded today.
Mrs May will now need the support of more than 50% of the 315 Conservative MPs to stay in office (158 in total). But even if she wins, if the margin of victory is small her authority may have been fatally wounded.
The ballot is from 6-8pm this evening (Wednesday), and the result will be announced shortly afterwards.
If the PM loses the vote, she would not be able to stand in the subsequent leadership contest.
Should Theresa May go?
Mrs May said this morning that she intends to battle on: “I will contest that vote with everything I’ve got.”
She added that a leadership contest would threaten the Brexit process: “A change of leadership in the Conservative Party now will put our country’s future at risk and create uncertainty when we can least afford it.”
But is her time up? Have your say:
How would the leadership election work?
Leadership candidates must be nominated by two Tory MPs. If only one candidate comes forward, he or she becomes leader.
If a number of would-be leaders are nominated, the list is whittled down to a shortlist of two in a series of votes by MPs.
The final pair then go to a postal ballot of all party members, with the position of leader – and Prime Minister – going to the victor.
The campaign would be expected to last around 12 weeks – although those calling for Mrs May to go believe it could be accomplished much quicker. Mrs May could remain in the post during the campaign period.
Who are the contenders to take over as Tory leader?
Bookmakers have Boris Johnson and Dominic Raab as joint favourites, followed by Michael Gove and Sajid Javed.
Although Cabinet colleagues like Mr Javed have voiced support for Mrs May, they could become contenders if she loses the no confidence vote.
Who would you like to see as the next PM? Have your say:
What next for Brexit?
It remains unclear what this means for the Brexit process, as Mrs May was in the process of seeking concessions from EU leaders on her divisive withdrawal agreement.
Speaking in Downing Street today, Mrs May said a new prime minister would have to scrap or extend Article 50, the mechanism taking Britain out of the EU on 29 March, “delaying or even stopping Brexit”.
What do you think should happen in the long run? Have your say: