She will now face a vital 12 hours as she fights tooth and nail to save her job.
The no confidence vote was triggered overnight after 48 Tories put in letters calling for the PM to go.
So, what happens next?
The PM has just made a speech outside Downing Street this morning where she vowed to fight the vote with everything she has.
She told MPs that she has been a member of the party for over 40 years and she passionately believed a better future lies ahead with her Brexit deal.
A divisive leadership battle now would only risk handing power to Jeremy Corbyn, or would delay Brexit altogether, she added.
"I have devoted myself unsparingly to this since I came to No10… I stand ready to finish the job," she said.
Later today the PM faces a tough grilling at Prime Minister's Questions, which will take place at midday.
It should be an easy open goal for Jeremy Corbyn to point out how weak she looks right now, and could be out of office by the end of the week.
Speech to 1922 committee
Mrs May will give a rally cry speech to her own MPs later tonight as they decide whether to back her or sack her.
A vote of confidence will take place between 6pm and 8pm, backbench boss Graham Brady announced.
She only needs to win by ONE vote to stay on as party leader and as PM.
But if she loses by just a few, she'll certainly be under pressure to quit anyway as she will have lost the confidence of most of the party.
The vote will be secret, and MPs don't need to worry about looking loyal to the PM.
They can vote however they like, regardless of whether they are a member of the government or not.
What happens if she wins?
The PM will stay in place, and rebels can't try and get her out of office using this method for at least another year.
She will go on fighting for her Brexit deal and trying to get it through the Commons.
What happens if she loses?
All bets will be off as the Tory party goes into another battle – this time for their next leader and PM.
Mrs May will technically stay in place as PM until another leader is picked out and approved by the Tories.
In the coming days MPs will put their names forward to replace her, and the names will be whittled down in rounds of voting.
The final two names will be put to the Tory party members, who will pick the next PM.
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