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 an emotional statement in Downing Street, Mrs May said she had “done my best” to honour the 2016 EU referendum result.
It would remain a matter of “deep regret” that she had been unable to deliver Brexit, she added.
But a new PM was “in the best interests of the country”.
Mrs May said she will continue to serve as prime minister while a Conservative leadership contest takes place.
She will step down as Tory leader on 7 June and a leadership contest is due to begin the following week.
Mrs May’s voice shook as she ended her speech saying: “I will shortly leave the job that it has been the honour of my life to hold.
“The second female prime minister, but certainly not the last.
“I do so with no ill will, but with enormous and enduring gratitude to have had the opportunity to serve the country I love.”