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.
LIZ TRUSS
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.
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 initially campaigned against Brexit but, after the referendum, said she had changed her mind.
She spent the first two years of Johnson's premiership as international trade secretary and was last year appointed as Britain's lead negotiator with the European Union.
Truss is now in charge of dealing with the EU over post-Brexit trade rules for Northern Ireland, where she has taken an increasingly tough line in negotiations.
She said on Monday that Johnson had her "100% backing" and she urged colleagues to support him.
JEREMY HUNT
The former foreign secretary, 55, finished second to Johnson in the 2019 leadership contest. He would offer a more serious and less controversial style of leadership after the turmoil of Johnson's premiership.
Over the last two years, 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". Hunt said he voted to oust Johnson in a confidence vote last month that the prime minister narrowly won.
Hunt supported remaining in the EU ahead of the 2016 vote. It is unclear whether he would feel the need to maintain a tough line against Brussels, to win the support of Conservative voters, or whether he could pursue a more pragmatic relationship to improve post-Brexit trade.
BEN WALLACE
Defence minister Ben Wallace, 52, has risen in recent months to overtake Truss as 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.
He supported remaining in the EU ahead of the referendum.
RISHI SUNAK
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 Johnson.
He was praised for a COVID-19 economic rescue package, including a costly jobs retention programme that averted mass unemployment.
But 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 Johnson, for breaking COVID 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.
Sunak voted to leave the EU in 2016.
SAJID JAVID
Javid was the first cabinet minister to resign in protest over accusations that 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, Javid has served in a number of cabinet roles, most recently as health minister. He resigned as 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.
Javid supported remaining in the EU "with a heavy heart and no enthusiasm", saying he feared the fallout from a leave vote would add to economic turbulence.
NADHIM ZAHAWI
The newly appointed finance minister impressed as vaccines minister when Britain had one of the world's fastest rollouts of COVID shots.
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. Zahawi said last week it would be a "privilege" to be prime minister at some stage.
He supported leaving the EU.
PENNY MORDAUNT
The former defence secretary was sacked by Johnson when he became prime minister after she endorsed his rival, Hunt, during the last leadership contest.
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, Mordaunt called the lockdown-breaking parties in government "shameful". She had previously expressed loyalty to Johnson.
Mordaunt campaigned to leave the EU in 2016.
TOM TUGENDHAT
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 Johnson and would offer his party a clean break with previous governments.
However, he is relatively untested because he has never served in cabinet.
He voted to remain in the EU.
SUELLA BRAVERMAN
A Brexit-backing Attorney General, Braverman has said she plans to run for the leadership. She was heavily criticised by lawyers during her tenure after the government sought to break international law over post-Brexit trade rules in Northern Ireland.
She campaigned to leave the EU and served as a junior minister in the Brexit department under May, but resigned in protest at the then prime minister's proposed Brexit deal, saying it did not go far enough in breaking ties with the bloc.
SOURCE: REUTERS
IMAGE SOURCE: REDDIT.COM
