While several possible successors are seen as frontrunners, there is no clear favourite -- and such contests are notoriously volatile.
Rishi Sunak
The UK's first Hindu finance minister, Sunak quit on Tuesday and declared he was standing three days later.
Long seen as Johnson's most likely successor, his prospects were dented earlier this year by questions over his private wealth and family's tax arrangements.
But a recent poll of Conservative members who will eventually vote for their new leader put him on top.
Sunak, 42, has a high profile on social media -- launching his bid in a slick video posted to Twitter -- and won plaudits for shoring up the economy during the pandemic.
But his apparent reluctance to embrace the immediate tax cuts being promised by rivals could harm his prospects.
Jeremy Hunt
Former foreign and health secretary Hunt, 55, lost to Johnson in 2019 but confirmed Saturday he will run again.
A supporter of remaining inside the European Union during the 2016 referendum, he has revealed Brexiteer Esther McVey will be his deputy if he wins.
A fluent Japanese speaker lacking Johnson's charisma, he has also vowed to cut corporation tax from 25 to 15 per cent.
Liz Truss
Foreign Secretary Truss confirmed her widely expected leadership bid on Sunday.
The 46-year-old is popular among Conservative members for her outspokenness.
But that has also stoked questions about her judgement, for instance when in February she encouraged Britons to fight in Ukraine.
Critics say her leadership posturing is too overt.
When she headed the Department for International Trade, some MPs dubbed it the "Department for Instagramming Truss" because of her prolific output on the social media site.
Sajid Javid
Javid, who also quit as health secretary Tuesday and had previously resigned as finance minister in 2020, announced his candidacy Saturday.The 52-year-old son of a Pakistani immigrant bus driver was previously a high-flying banker.
Like Sunak, he also faces questions about his personal wealth and tax affairs but tried to brush them off in media interviews Sunday, promising a raft of tax cuts.
Nadhim Zahawi
Newly appointed finance minister Zahawi was praised for overseeing Britain's pandemic vaccines rollout, before helming the education department.
The 55-year-old is a former refugee from Iraq who came to Britain as a child speaking no English. Before entering politics, he co-founded the prominent polling company YouGov.
But his private wealth has also drawn adverse attention, including when he claimed parliamentary expenses for heating his horse stables.
His fledgling campaign was endangered Sunday after newspapers reported he is being investigated by UK tax authorities, though he has denied wrongdoing.
Tom Tugendhat
The prominent backbencher who chairs parliament's influential Foreign Affairs Committee was the first to launch his bid.A former army officer who served in the Middle East, he is also a hawk on China and has been critical of the government's handling of the troop withdrawal from Afghanistan.
The 49-year-old is pitching himself as a "clean start" after Johnson's turbulent three-year tenure.
Penny Mordaunt
Mordaunt, 49, the first woman to have been UK defence secretary and currently a trade minister, joined the contest in a video posted to social media Sunday.
A strong Brexit supporter and key figure in the 2016 "Leave" campaign, she has been tipped as a potential unity candidate who could draw support from the Conservative party's warring factions.
Suella Braverman
Attorney general and arch-Brexiteer Braverman declared her campaign in mid-week media interviews.
The 42-year-old is popular within the party for her Euroscepticism.
She is one of 28 so-called "Spartan" Tory MPs, who refused to back ex-prime minister Theresa May's Brexit deal on all three occasions it was voted on in parliament.
Grant Shapps
Transport Secretary Shapps emerged onto the crowded field on Saturday, also promising tax cuts and competent government.The 53-year-old is seen as an effective communicator and campaigner but is considered a long shot for the top job.
Kemi Badenoch
Former equalities minister Badenoch, who resigned Wednesday, has one of the lowest profiles of the current contenders and is another unlikely victor.The 42-year-old has promised to tell "the truth", saying people are "exhausted by platitudes and empty rhetoric".
Rehman Chishti
An MP for Gillingham and Rainham in southeast England, the relatively unknown Chishti announced his candidacy on Sunday evening, speaking of "aspirational conservatism, fresh ideas, fresh team for a fresh start" on Twitter.
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