Boris leads the field as the race to be next British prime minister gets under way
Here are some of the Conservatives who plan to put themselves forward or are widely expected to run for the leadership:
BORIS JOHNSON (54)
The face of the official campaign to leave the EU, Mr Johnson resigned as foreign minister in July in protest at Mrs May's handling of the exit negotiations.
He set out his pitch to the membership in a speech at the party's annual conference in October. He called on the party to return to its traditional values of low tax and strong policing.
He is the bookmakers' favourite to succeed Mrs May.
ESTHER MCVEY (51)
The pro-Brexit former television presenter, who resigned as work and pensions minister in November in protest at Mrs May's exit deal with the EU, has said she plans to run.
RORY STEWART (46)
A former diplomat who once walked 9,600km across Iran, Afghanistan, Pakistan, India and Nepal, Mr Stewart was promoted to international development secretary this month. Educated at the exclusive Eton College, he backed remaining in the EU in the 2016 referendum. He opposes a no-deal exit and has been a vocal advocate of Mrs May's deal with Brussels.
MICHAEL GOVE (51)
Mr Gove, one of the highest-profile Brexit campaigners during the 2016 referendum, has had to rebuild his cabinet career after falling early to Mrs May in the contest to replace David Cameron. Seen as one of the most effective members of cabinet in bringing forward new policies, the environment minister has become a surprise ally to Mrs May and has backed her Brexit strategy.
He has not yet said whether he plans to run.
JEREMY HUNT (52)
He replaced Mr Johnson as foreign minister in July and has urged the Conservative membership to set aside their differences over Brexit and unite against a common foe - the EU.
Mr Hunt voted to remain in the EU in the referendum. Asked at a lunch with journalists in parliament if he planned to run for leader, he said: "Wait and see."
ANDREA LEADSOM (56)
A pro-Brexit campaigner, Ms Leadsom made it to the last two in the 2016 contest to replace Mr Cameron. She withdrew after a backlash to an interview in which she said being a mother gave her more of a stake in the future of the country than her rival Theresa May. She quit as leader of the House of Commons on Wednesday, saying she did not believe the government's approach would deliver on the Brexit referendum result.
DOMINIC RAAB (45)
Mr Raab quit as Brexit minister last year in protest at Mrs May's draft exit agreement saying it did not match the promises the Conservative Party made in the 2017 election. He served only five months as head of the Brexit department. Asked if he would like to be prime minister, he said: "Never say never."
PENNY MORDAUNT (46)
Ms Mordaunt is one of the last remaining pro-Brexit members of Mrs May's cabinet. She became Britain's first female defence secretary this month.
A royal navy reservist, Ms Mordaunt was previously international development minister.