Forty-eight letters from MPs have been written to the chairman of the backbench 1922 Committee, Sir Graham Brady, triggering a no confidence vote.
The ballot will be held on Wednesday evening after the threshold of letters from Tory MPs needed to trigger the contest was exceeded.
- What is the historic 1922 Committee?
The committee of all backbench Conservative MPs, also known as "the '22" meets weekly when the House of Commons is sitting.
When the party is in opposition, frontbenchers are allowed to attend, although the leader cannot.
Members are said to bang their desks in approval when the chief arrives, when the party in in government and the leader attends.
- Why is it called the 1922 committee?
The committee takes its name from a meeting of Conservative MPs on October 19 1922.
The MPs successfully ended the party’s coalition with the Liberals, bringing down the government of David Lloyd George.
The resulting general election was won by the Tories.
- Who is in charge?
The '22 has an 18-member executive committee with a chairman, usually a senior MP, elected by committee members.
The incumbent chairman is Sir Graham Brady, MP for Altrincham and Sale West, who was appointed in May 2010.
- What does the committee do?
The committee keeps the party informed of the backbenchers' mood and opinion on party business.
The chairman has considerable influence within the Parliamentary Conservative Party and oversees the election of a new leader.
A leader who loses the confidence of the '22 is likely to find themselves in a precarious position as Cabinet ministers take the committee’s views seriously.
- What has the '22 achieved?
In October 2003 Iain Duncan Smith was dumped as leader after he lost a confidence vote by 75 votes to 90.
- What is all the talk about 48 letters?
One way in which a group of rebels could mount a leadership challenge to Mrs May is by calling a vote of no confidence, which would require 15% of of the parliamentary party to back the idea.
This would mean 48 letters would have to be sent to Sir Graham supporting the idea and then a simple majority (158) supporting the motion of no confidence.