Below are some of the candidates who could replace Johnson if he resigned or was ousted by his lawmakers:

RISHI SUNAK

The finance minister is the bookmakers' favourite to be Britain's next prime minister. He has been praised for a rescue package for the economy during the coronavirus pandemic, including a jobs retention programme, which prevented mass unemployment, that could cost as much as 410 billion pounds ($559 billion).

However, support for the 41-year-old has slipped in recent months among rank-and-file Conservative members, who would vote to elect which of the final two candidates would win the party's top job in a leadership election.

Sunak's tax-and-spend budget in October put Britain on course for its biggest tax burden since the 1950s, undermining his claims to favour lower taxes.

The former Goldman Sachs banker has only offered Johnson tepid support since he apologised for the lockdown parties. Sunak abruptly ended a broadcast interview on Tuesday when asked if he "unequivocally" supported him.

Almost half of Conservative members believe that Sunak would make the best future leader, the highest support of any potential candidate, a poll by Opinium published this week found.

LIZ TRUSS

The foreign secretary is the darling of the Conservatives' grassroots and regularly tops the poll 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, evoking a famous 1986 image of Britain's first female prime minister, Margaret Thatcher, who was also captured in such a pose.

The 46-year-old spent the first two years of Johnson's premiership as international trade secretary, championing Brexit, and last month was appointed as Britain's lead negotiator with the European Union.

Truss is also known for bizarre political interventions. A 2014 speech to the Conservatives annual conference went viral after she angrily declared: "We import two thirds of our cheese. That. Is. A. Disgrace."

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.

Over the last two years, Hunt has used his experience as a former health secretary to chair the health select committee and has not been tarnished by having served in the current government.

Hunt told The House political magazine in an interview this week that his ambition to become prime minister "hasn't completely vanished".

SAJID JAVID

The current health minister, 52, came fourth in the last leadership contest.

Javid, a second-generation immigrant of Pakistani heritage, has served a number of cabinet roles, including as finance minister, and scores consistently well in polls of party members.

He is a minister who is described by several public servants who worked for him as decisive and clear.

DOMINIC RAAB

The deputy prime minister has held top roles in government, including serving as foreign minister and Brexit Secretary.

Raab, 47, stood in for Johnson as acting prime minister after Johnson caught COVID-19 early in the pandemic and was admitted to intensive care.

But he was heavily criticised for remaining on holiday during the Taliban's recapture of Afghanistan in August last year and was demoted to justice secretary. ($1 = 0.7330 pounds)