Her successor will have to try to complete Brexit — a task that May failed to deliver. While she succeeded in striking a divorce deal with the European Union, the plan was defeated three times in Parliament.
The EU extended Britain's departure date to October 31 but there still is no consensus among MPs.
The best-known contestant is former Foreign Secretary Boris Johnson, who has said he will take Britain out of the EU on October 31 even if no deal has been reached with EU leaders. Johnson's willingness to back a no-deal Brexit is causing ripples.
Another contender, International Development Secretary Rory Stewart, said that he could not serve in a Cabinet under Johnson if Johnson wins. Stewart says he could not work for a leader who is comfortable with the idea of a no-deal Brexit.
The field is likely to grow to about a dozen candidates, with a winner expected to be chosen by mid or late July. Senior Conservatives including Home Secretary Sajid Javid and Foreign Secretary Jeremy Hunt are among those considering a run.
The party chooses its leaders in a two-step process. First MPs vote to establish two top contenders, then those names are submitted to a nationwide vote by about 120,000 party members.
The UK's next general election will be held in 2022 unless a government collapse speeds up the timetable.
- AP