Sir Graham Brady, Chairman of the 1922 Committee, has received at least 48 letters of no confidence in the Prime Minister.
This means Mrs May will face a vote of no confidence, because the 15% threshold has now been reached.
Meanwhile, Mrs May has said she will “contest that vote with everything Ive got”.
Theresa May no-confidence vote: What time is vote of no confidence? When is result? (Pic: GETTY)
When is the vote of no confidence?
The vote of no confidence will take place tonight, between 6pm and 8pm.
Brady has said the votes will be counted “immediately afterwards and an announcement will be made as soon as possible”.
Sources have said an announcement will be made by 10pm, report The Guardian.
Meanwhile, if she loses the vote it could take up to 12 weeks for a leadership contest to take place.
However, it would be Sir Graham who would be overseeing the contest and setting appropriate timetables.
Those MPs who have submitted a vote of no confidence think it should achieved far sooner than that.
Mrs May will address the 1922 committee, which is comprised of all backbench Tory MPs, at 5pm.
Theresa May no-confidence vote: The vote takes place today (Pic: REUTERS)
Theresa May no-confidence vote: A no confidence vote has been declared (Pic: REUTERS)
Then MPs will vote in the House of Commons to either support or oppose the Prime Minister.
Ultimately they will be voting on whether or not they have confidence in their party leader.
While only 48 MPs were needed to trigger the vote, it will require a simple majority of 158 to depose the Prime Minister.
If she wins the vote she would get immunity from formal challenges for a year.