The prime minister is facing a vote of no confidence from Conservative MPs after the threshold of letters needed to trigger a contest was exceeded.
More than 48 Conservative MPs have submitted letters calling for her to go to the chairman of the backbench 1922 Committee, Sir Graham Brady.
Sir Graham said a ballot would be held between 6pm and 8pm on Wednesday in the Commons.
Mrs May is expected to speak with individual Tory MPs during the day before addressing the 1922 Committee at 5pm, immediately before the crucial vote.
How does the vote work?
It’s a secret ballot, held inside a room somewhere in the Houses of Parliament. The vote will be open for around two to three hours and MPs must physically turn up to mark their X in the box - either declaring confidence, or no confidence, in Theresa May.
MPs can vote by proxy with the permission of the chair.
When will the result be announced?
Quickly, most likely the same day - probably the evening of the vote closing.
To win, Theresa May needs the support of 158 Conservative MPs for a simple majority of one (if every eligible MP votes). That’s enough for her to stay on if she wishes.
Victory for Mrs May would also mean she cannot be challenged in this way again for 12 months.
What happens if she loses?
Well, she’ll resign as Conservative Party leader immediately BUT will remain as Prime Minister, living in Number 10, until the Tories elect her replacement.
You may remember David Cameron stayed in Downing Street even after he announced he was resigning after the EU referendum, only leaving once the Tories had elected Theresa May.
The party will then move fast to pick a new leader. Any MP can put their name forward and their colleagues will whittle the names down to two. The final pair will then be put forward to Tory party members to choose.
My understanding is that the party would move as fast as possible to elect a new leader, allowing two weeks for MPs to pick two names and around three weeks for members to pick one.
But but but...there is absolutely no guarantee she would actually lose a confidence vote. After all, it has taken many weeks for Mrs May's critics to summon the 48 letters (15% of the party's MPs) needed to challenge her.
Furthermore there are plenty of Tories who think it lunacy to change leader at this crucial time in the Brexit process, and there is no clear front-runner to replace her. And remember, if she wins, she’s got guaranteed immunity for a whole year if she wants it.
If you come at the Queen, and all that...