Mrs. 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.
Mrs. May 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.
Nominations open from 10:00 BST on Monday and close at 17:00 BST that day.
Mrs. May remains acting leader while the contest takes place.
The UK was originally meant to leave the European Union on 29 March. That was then pushed back to 12 April and eventually 31 October after Mrs. May failed to get MPs’ to approve her deal.
She announced her resignation, saying she had done everything she could to try to persuade MPs’ to support the withdrawal deal which she had negotiated with the European Union but it was now time for a new prime minister to try to deliver Brexit.