Prime Minister Theresa May announced her resignation in an emotional address on Friday, ending a dramatic three-year tenure of near-constant crisis over Brexit and increasing the likelihood of Britain crashing out of the EU later this year.
"It is and will always remain a matter of deep regret to me that I have not been able to deliver Brexit," May, her voice breaking and close to tears, said outside her Downing Street office.
The 62-year-old leader said she would step down as head of the Conservative Party on June 7.
She will remain as prime minister in a caretaker role until a replacement is elected by the party before July 20.
Tory MPs will hold a series of votes to whittle the contenders down to a final two that will be put to the party's more than 100,000 members.
Former foreign secretary and gaffe-prone Brexit cheerleader Boris Johnson is the membership's favourite, but numerous Conservative MPs are thought to hold serious reservations about his suitability for the top job.
Johnson, who has repeatedly said Britain should not fear a no-deal Brexit, thanked May for her "stoical service".
"It is now time to follow her urgings: to come together and deliver Brexit," he said on Twitter.
May was the surprising victor in a 2016 leadership contest to replace then prime minister David Cameron after he resigned in the aftermath of the Brexit referendum.
Despite having campaigned to stay in the EU, she embraced the cause with the mantra "Brexit means Brexit".
However the decision to hold a disastrous snap election in June 2017, when she lost her parliamentary majority, left her stymied.
Her dismally dysfunctional government saw 36 ministerial resignations – a modern record.
May will leave office without any significant achievements – other than her bungled handling of Brexit, according to political analysts.
"She doesn't really have a legacy," said Simon Usherwood, from the University of Surrey's politics department. "I think anybody in her position would have had great difficulty."
Others were more brutal in their assessment.
"It was only an impossible job because she made it one," said Tim Bale of Queen Mary University of London.