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.
The winner of the contest is expected to be announced in the week of 22 July.
Mrs May remains acting party leader during the leadership election process.
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 UK was originally meant to leave the European Union on 29 March but that was then pushed back to 12 April and eventually 31 October.
When Mrs May announced her resignation, she said it was time for a new prime minister to try to deliver Brexit.
The end of her time as party leader will come in a private exchange of letters with Charles Walker and Dame Cheryl Gillan, the joint acting chairmen of the 1922 Committee of backbench Conservative MPs.
There will then be a call for candidates issued at 17:00 BST, with nominations opening on Monday from 10:00 and closing at 17:00 BST that day.
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