Below is a summary of some of those who could be in the frame to replace him. However, there is no clear favourite, and they are not listed in order of likely prospects.
The foreign secretary is the darling of the ruling Conservative Party’s grassroots and has regularly topped polls of party members carried out by the website Conservative Home.
Ms. Truss has a carefully cultivated public image and was photographed in a tank last year, echoing a famous 1986 photo of Britain’s first female prime minister, Margaret Thatcher.
The 46-year-old spent the first two years of Mr. Johnson’s premiership as international trade secretary, championing Brexit, and last year was appointed as Britain’s lead negotiator with the European Union.
Ms. Truss said on Monday that Mr. Johnson had her “100-per-cent backing” and she urged colleagues to support him.
The former foreign secretary, 55, finished second to Mr. Johnson in the 2019 leadership contest. He would offer a more serious and less controversial style of leadership after the turmoil of Mr. Johnson’s premiership.
Over the past two years, Mr. Hunt has used his experience as a former health secretary to chair Parliament’s health select committee and has not been tarnished by having served in the current government.
Earlier this year, he said his ambition to become prime minister “hasn’t completely vanished.” Mr. Hunt said he voted to oust Mr. Johnson in a confidence vote last month that the Prime Minister narrowly won.
Defence Minister Ben Wallace, 52, has risen in recent months to be the most popular member of the government with Conservative Party members, according to Conservative Home, thanks to his handling of the Ukraine crisis.
A former soldier, he was mentioned in dispatches in 1992 for an incident in which the patrol he was commanding captured an Irish Republican Army guerrilla unit suspected of trying to carry out a bomb attack on British troops.
He began his political career as a member of Scotland’s devolved assembly in May, 1999, before being first elected to the Westminster Parliament in 2005.
He was security minister from 2016 until taking on his current role three years later, winning plaudits as his department evacuated British nationals and allies from Afghanistan last year, and for sending weapons to Kyiv.
Mr. Sunak, who resigned as finance minister on Tuesday saying the British public “rightly expect government to be conducted properly, competently and seriously,” was until last year the favourite to succeed Mr. Johnson.
He was praised for a COVID-19 economic rescue package, including a costly jobs retention program that averted mass unemployment.
But Mr. Sunak later faced criticism for not giving enough cost-of-living support to households. Revelations about his wealthy wife’s non-domiciled tax status and a fine he received, along with Mr. Johnson, for breaking COVID-19 lockdown rules have damaged his standing.
His tax-and-spend budget last year put Britain on course for its biggest tax burden since the 1950s, undermining his claims to favour lower taxes.
Mr. Javid was the first cabinet minister to resign in protest over accusations that Mr. Johnson misled the public over what he knew about sexual-harassment allegations against a Conservative lawmaker.
A former banker and a champion of free markets, Mr. Javid has served in a number of cabinet roles, most recently as health minister. He resigned as Mr. Johnson’s finance minister in 2020.
The son of Pakistani Muslim immigrant parents, he is a Thatcher-admirer and finished fourth in the 2019 leadership contest to replace former prime minister Theresa May.
The newly appointed finance minister impressed as vaccines minister when Britain had one of the world’s fastest rollouts of COVID-19 jabs.
Mr. Zahawi’s personal story as a former refugee from Iraq who came to Britain as a child sets him apart from other contenders.
He co-founded polling company YouGov before entering Parliament in 2010. His last job was as education secretary. Mr. Zahawi said last week that it would be a “privilege” to be prime minister at some stage.
The former defence secretary was sacked by Mr. Johnson when he became Prime Minister after she endorsed his rival Mr. Hunt during the last leadership contest.
Ms. Mordaunt was a passionate supporter of leaving the European Union and made national headlines by taking part in a now-defunct reality TV diving show.
Currently a junior trade minister, Ms. Mordaunt called the lockdown-breaking parties in government “shameful.” She had previously expressed loyalty to Mr. Johnson.
The chair of Parliament’s Foreign Affairs Committee, and a former soldier who fought in Iraq and Afghanistan, has already indicated he would run in any leadership contest.
He has been a regular critic of Mr. Johnson and would offer his party a clean break with previous governments.
However, he is relatively untested because he has never served in cabinet.
A Brexit-backing Attorney-General, Ms. Braverman has indicated she will run for the leadership. She was heavily criticized by lawyers during her tenure after the government sought to break international law over post-Brexit trade rules in Northern Ireland.
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