Theresa May will officially step down as the leader of the Conservative Party on Friday, but will remain as prime minister until her successor is chosen.
She announced her resignation two weeks ago, saying it was a matter of deep regret that she had been unable to deliver Brexit.
Eleven Conservative MPs are vying to replace her as party leader and, ultimately, prime minister.
Mrs. May remains acting party leader while the contest takes place.
Meanwhile, the Conservatives fell to third place in the Peterborough by-election, behind winners Labour and the Brexit Party in second place, in what is traditionally a Tory-Labour marginal seat.
Mrs. May’s time as leader has been dominated by Brexit, with her party divided over the issue, and the failure to get her deal through Parliament.
The final two will be put to a vote of members of the wider Conservative Party, with a winner expected to be announced in the week of 22 July.
While the contest does not officially start until Mrs. May steps down, candidates have already been jostling for position.