Nominations are set to close at 6 p.m. (1700GMT). Candidates need support from at least 20 Conservative lawmakers to be on the ballot for run-off votes, which will start Wednesday.
These are the 10 people who have thrown their hats into the ring so far:
RISHI SUNAK, FORMER TREASURY CHIEF
Sunak, 42, is the best-known of the party's potential leaders and a favorite of bookmakers to succeed Johnson.
His public profile rose after he became the U.K.'s Treasury chief in 2020, a job that entailed overseeing the billions of pounds handed out during the coronavirus pandemic to help businesses and workers.
But he has faced criticism for not doing enough to ease the country's growing cost-of-living crisis. His popularity also took a dive after he police fined him for attending one of the government's lockdown-flouting parties at Downing Street, home to the prime minister's office and official residence.
Sunak says the promises to cut taxes made by many rival candidates are ``not credible'' and that he would only reduce taxes after inflation is under control.
LIZ TRUSS, FOREIGN SECRETARY
Truss, 46, took on the high-profile Cabinet post in September after serving as trade minister. She is also the U.K.'s lead negotiator with the European Union on issues following Brexit.
Truss, the U.K.'s top diplomat, has made no secret of her leadership ambitions and has sought to emulate former Conservative Prime Minister Margaret Thatcher in carefully curated social media photos. If selected, she says she wants to cut taxes ``from day one'' and reverse an income tax hike.
PENNY MORDAUNT, TRADE MINISTER
Mordaunt, 49, is popular with many Conservatives and is another bookmakers' favorite to win.
She was the first woman to serve as U.K. defense secretary, although Johnson dismissed her shortly after he became prime minister in 2019 because she had backed another candidate for party leader, Jeremy Hunt.
Mordaunt, a former reality TV contestant, played a prominent role in the pro-Brexit campaign in 2016.
JEREMY HUNT, FORMER CABINET MINISTER
Hunt, who has served as both health and foreign secretary, ran against Johnson in the 2019 leadership race. He lost heavily, and was dumped from the Cabinet.
Hunt, 55, has remained a member of Parliament and kept himself in the public eye by grilling the government's pandemic policies as head of the Health and Social Care Select Committee.
TOM TUGENDHAT, COMMONS FOREIGN AFFAIRS COMMITTEE CHAIR
Tugendhat, a 48-year-old lawmaker and former soldier, has never held a Cabinet post but heads the influential Foreign Affairs Committee in Parliament. He is seen as a centrist and has billed his candidacy as an opportunity for a clean start.
He said he is consulting a ``broad coalition'' of colleagues to bridge the divides in government.
KEMI BADENOCH, FORMER EQUALITIES MINISTER
Badenoch, a former banker, says she wants to lower taxes and lead a ``limited government focused on essentials.''
The 42-year-old lawmaker was initially considered an outsider for the race, but an endorsement from party heavyweight Michael Gove has boosted her profile.
SAJID JAVID, FORMER HEALTH SECRETARY
Javid, 52, has wide experience in government, having held the posts of health secretary, Treasury chief and home secretary as well as leading the British government departments for business, culture and housing.
The son of a bus driver who arrived in England from Pakistan, Javid made it to the final four in the contest to replace Theresa May as Tory leader in 2019.
NADHIM ZAHAWI, TREASURY CHIEF
Zahawi, 55, came to prominence as vaccine minister during the pandemic. A co-founder of the market research firm YouGov, Zahawi was elected to Parliament in 2010.
He was born in Iraq to a Kurdish family and came to the U.K. as a child when his parents fled Iraq under Saddam Hussein.
SUELLA BRAVERMAN, ATTORNEY GENERAL
Braverman is a lawmaker and lawyer who became England's attorney general in 2020.
A euroskeptic and supporter of Britain's withdrawal from the European Union, Braverman, 42, says she wants to deliver ``all of the great opportunities of Brexit`` _ including a tougher line on immigration and leaving the jurisdiction of the European Court of Human Rights.
REHMAN CHISHTI, FOREIGN OFFICE MINISTER
Chishti, a former lawyer, is the least known of the contenders.
Born in Pakistan, he has an unusual political background, having previously stood as a candidate for the opposition Labour Party in the 2005 general election. The 43-year-old joined the Conservatives in 2007.