With Boris Johnson out, who will be Britain's next PM?

London's former mayor Boris Johnson, the runaway bookmakers' favourite to become Britain's next prime minister, abruptly pulled out of the race on Thursday in a shock announcement less than a week after leading the campaign to exit the EU.

Johnson's announcement, to audible gasps from a roomful of journalists and supporters, was the biggest political surprise since Prime Minister David Cameron quit on Friday, the morning after losing the referendum on British membership in the bloc.

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Conservative lawmakers said Johnson may have been undone by supporters of Cameron exacting revenge for his decision to defy the prime minister and back the Leave campaign.

"He who lives by the sword, dies by the sword," said one lawmaker, describing internal party conflict on condition of anonymity. The lawmaker told Reuters that Johnson had realised his bid would fail after lawmakers defected from his campaign overnight.

Boris Johnson drops out of U.K. leadership race

8 years ago

Duration 2:24

After leading the U.K. out of the European Union, former London mayor Boris Johnson has decided he's not up for dealing with the Brexit aftermath.

Johnson became the latest political casualty of a civil war in the ruling party unleashed by Cameron's decision to hold the referendum on membership in the EU, an issue that divided the Conservatives for decades and now divides the country.

While Johnson may be out of the running to replace Cameron, there are other candidates who have already thrown their hats into the ring for Britain's biggest job. 

Stephen Crabb​

Work and Pensions Secretary Stephen Crabb, 43, declared his candidacy on Wednesday, the first to do so. He pitched himself as a "blue collar" candidate — one who had worked up from a job in a local shop in an effort to distinguish himself from the "Eton elite."

Crabb is running with Business Secretary Sajid Javid — who would serve as his finance minister — on what has been dubbed a blue-collar ticket. Both men come from working-class backgrounds, in contrast to Johnson and Prime Minister Cameron.

 Although Crabb backed the "remain" side in the referendum, he promises to deliver a "negotiated exit" from the EU.

Theresa May

Theresa May, 59, the interior minister who campaigned to remain in the EU, announced her candidacy to lead the party on Thursday and is the new favourite to succeed Cameron.

Home Secretary May is seen as competent, experienced and a candidate who could appeal to Conservatives who weren't fond of Johnson.

Notably, May launched a thinly veiled attack on Johnson in a Times column, portraying herself as representative of ordinary Britons, and more understanding of their lives, than Johnson.

Despite having campaigned against Brexit, she promised to deliver the EU withdrawal voters had demanded.

"Brexit means Brexit," she told a news conference. "The campaign was fought, the vote was held, turnout was high and the public gave their verdict. There must be no attempts to remain inside the EU, no attempts to rejoin it through the back door and no second referendum."

Michael Gove

Justice Secretary Michael Gove, 48, one of the main campaigners to take Britain out of the EU and ally of Johnson announced Thursday he would run to become prime minister.

Gove, a close friend of Cameron, had previously said he would back Johnson. He tried to explain his 180-degree turnabout in a statement Thursday, saying he had "wanted to help build a team behind Boris Johnson."

"But I have come, reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead," Gove said. "I have, therefore, decided to put my name forward for the leadership."

Gove, who is liked within the party but is less popular with the public, had long disclaimed any ambition to be Britain's leader.

In 2012, he said he was willing to "sign a piece of parchment in my own blood saying I don't want to be prime minister." Asked if he would run earlier this month, he said: "Count me out."

Andrea Leadsom

British lawmaker Andrea Leadsom, 53, a prominent campaigner for Britain to leave the European Union, said on Thursday she would be standing as a candidate to replace Cameron as Conservative leader.

"My personal view very genuinely is the next leader is someone who has to deliver on the promise of the referendum because they can see the huge advantages and they believe in them," Leadsom, a junior minister in the energy department, told Sky News.

"I think it's very difficult for somebody who campaigned to stay in, who thinks that there will be disaster if we leave to suddenly turn it around and start believing we can make a go of it."

Liam Fox

Liam Fox, 54, a former defence secretary who backed Brexit, also formally announced his candidacy Thursday. Fox, on the anti-EU wing of the party, had to resign from the government in 2011 after allowing a friend access to official meetings and overseas visits.