She announced her resignation two weeks ago, saying it was a matter of deep regret that she had been unable to deliver Brexit.
However, under United Kingdom electoral commission rules, May will continue in her post in an acting capacity until 23 July when the party picks a successor, who will also become Britain's next prime minister.
The race is on to become leader of the Conservative Party and Britain's new prime minister.
Brexit Party leader Nigel Farage reacts as results are announced at the counting centre for the European Elections for the South East England region.
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.
She became prime minister in the aftermath of the 2016 Brexit referendum which swept away her predecessor David Cameron.
Although she had voted to remain, May insisted from day one of her tenure at Downing Street that the will of the British people must be respected.
May will remain in office as a powerless caretaker prime minister until the leadership election is concluded likely late next month.
The final straw came when compromises offered to attract more support for the Brexit deal failed to sway opposition politicians and enraged members of her own party.
Nearly a dozen Conservative lawmakers are already jostling to replace May in a contest that formally opens Monday, vowing to succeed where she failed and renegotiate Britain's divorce deal with the European Union.
Uber Air flying taxi service to be trialled in Melbourne
Macquarie's role would be around "the development and electrification of the skyports that will support" Uber's air taxis. Anderson said the company hopes to begin test flights next year, with an eye to open the service to commuters from 2023.
Cristiane scores hat trick in Brazil’s win over Jamaica
Marta, a 33-year-old forward, missed Sunday's match with a left thigh injury she suffered last month in training. I hope we can qualify for the knockout stage because these two goals are meant to help us do just that".
Former Red Sox star David Ortiz shot in Dominican
Once doctors had stabilized him he was flown to Massachusetts General Hospital in Boston where he underwent a second surgery. Ortiz has been alert, has spoken to his family and "even flashed that smile", his media assistant, Leo Lopez , told ESPN .
The contest should be completed in the week commencing July 22.
Nominations must be submitted by 5pm London time on Monday.
Boris Johnson, the bookies' favourite to inherit the Tory crown, also used the defeat to forward his campaign.
"I believe it was right to persevere even when the odds against success seemed high but it is now clear to me that it is in the best interests of the country for a new prime minister to lead that effort", she said.
Front runners include former London Mayor and Foreign Secretary Boris Johnson, Foreign Secretary Jeremy Hunt, International Development Secretary Rory Stewart, and Home Secretary Sajid Javid.
Over the following days more than 300 Conservative MPs at Westminster will vote on the long list of names, using a process of elimination until only two names are left.
Gove said he hoped that the disclosure would not prevent him being appointed the next Prime Minister. Will her replacement be able to finally deliver Brexit?
The Guardian newspaper said that if Johnson wins the race, he could face an immediate vote of no confidence in the House of Commons after he made a statement insisting Britain must leave the European Union on October 31, with or without a deal.
Her promise to leave the EU's institutions and end free movement of workers delighted eurosceptic MPs, but caused dismay among many pro-Europeans.
The main losers of the Brexit party surge have been the ruling Conservatives.
May 21, 2019: May offers a concession to lawmakers, giving them the chance to vote on whether to hold a new referendum on the country's membership in the European Union - but only if they back her thrice-rejected Brexit agreement.