British Prime Minister Theresa May says she will quit on June 7, triggering a contest next month that will bring a new leader to power who is likely to push for a cleaner break with the European Union.
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Below are the Conservative MPs who have either said they plan to put themselves forward or are widely expected to run for the leadership:
Planning to run:
Boris Johnson, 54
The face of the official campaign to leave the European Union, Johnson resigned as foreign minister in July in protest at May's handling of the exit negotiations.
Johnson set out his pitch to the membership in a speech at the party's annual conference in October - some members queued for hours to get a seat. He called on the party to return to its traditional values of low tax and strong policing.
He is the bookmakers' favourite to succeed May.
Esther Mcvey, 51
The pro-Brexit former television presenter, who resigned as work and pensions minister in November in protest at May's exit deal with the EU, has said she plans to run.
Rory Stewart, 46
A former diplomat who once walked 9600km across Iran, Afghanistan, Pakistan, India and Nepal, Stewart was promoted to International Development Secretary this month.
Expected to run:
Michael Gove, 51
Gove, one of the highest-profile Brexit campaigners during the 2016 referendum, has had to rebuild his cabinet career after falling early to May in the contest to replace David Cameron, who resigned the day after losing the referendum.
Seen as one of the most effective members of cabinet in bringing forward new policies, the high-energy environment minister has become a surprise ally to May and has backed her Brexit strategy.
He has not yet said whether he plans to run.
Jeremy Hunt, 52
Hunt replaced 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.
Hunt voted to remain in the EU in the referendum. He served six years as Britain's health minister, a role that has made him unpopular with many voters who work in or rely on the state-run, financially stretched National Health Service.
Andrea Leadsom, 56
A pro-Brexit campaigner, Leadsom made it to the last two in the 2016 contest to replace 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.
Dominic Raab, 45
Raab quit as May's Brexit minister last year in protest at her draft exit agreement saying it did not match the promises the Conservative Party made in the 2017 election. Raab served only five months as head of the Brexit department.
Sajid Javid, 49
Javid, a former banker and a champion of free markets, has served a number of cabinet roles and scores consistently well in polls of party members. A second-generation immigrant of Pakistani heritage, he has a portrait of late Conservative Prime Minister Margaret Thatcher on his office wall.
David Davis, 70
Davis, a leading eurosceptic, was appointed Brexit minister to lead negotiations with the EU in July 2016 but resigned two years later in protest at May's plans for a long-term relationship with the bloc.
Penny Mordaunt, 46
Mordaunt is one of the last remaining pro-Brexit members of May's cabinet. She became Britain's first female defence secretary this month.
Amber Rudd, 55
Rudd resigned as interior minister last year after facing outrage over her department's treatment of some long-term Caribbean residents wrongly labelled illegal immigrants.
Matt Hancock, 40
Health minister Hancock, a former economist at the Bank of England, supported 'Remain' in 2016. First elected to parliament in 2010, he has held several ministerial roles.
Justine Greening, 50
The former education minister told ITV she would consider running. Greening supports a second Brexit referendum. Many thought she might join several of her colleagues in quitting the party to form a pro-EU group in parliament earlier this year.
Liz Trus, 43
Chief Secretary to the Treasury, Truss has held several roles in government including environment minister and justice minister. She backed 'Remain' in 2016 but has said she has since changed her mind on Brexit.
Graham Brady, 51
Brady is chair of the 1922 Committee of Conservative lawmakers. "It would take an awful lot of people to persuade me. I'm not sure many people are straining at the leash to take on what is an extraordinarily difficult situation," he told BBC Radio.
Kit Malthouse, 52
A former deputy Mayor of London, Malthouse became a Member of Parliament in 2015. He is a junior housing minister and helped author the so-called Malthouse Compromise plan to replace the unpopular Irish backstop in Britain's EU exit deal with alternative arrangements to avoid a hard border.
James Cleverly, 49
Cleverly was appointed a junior Brexit minister last month, having previously been deputy chair of the Conservative Party. He had a career in publishing before being elected to parliament in 2015.
The Sun reported he was planning to run for leader.