Theresa May has said she will quit as Conservative leader on 7 June, paving the way for a contest to decide a new prime minister.
In a statement in Downing St, she said that it would remain a matter of "deep regret" that she had been unable to deliver Brexit.
The prime minister has faced a backlash from her MPs against her latest Brexit plan, which included concessions aimed at attracting cross-party support. Parliament has rejected Mrs May's withdrawal agreement three times.
She broke down in tears as she said serving as prime minister was the honour of her life.