The shock announcement comes less than a week after leading the campaign to pull the country out of the European Union.

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.

Johnson’s withdrawal makes Theresa May, the interior minister who backed remaining in the EU, the new favourite to succeed Cameron.

She announced her own candidacy earlier on Thursday, promising to deliver the EU withdrawal voters had demanded, despite having campaigned for the other side.

“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.”

Johnson, whose support of the Leave cause was widely seen as delivering its victory, saw his bid suddenly crumble after his Brexit campaign ally, Justice Secretary Michael Gove, withdrew his backing and announced his own leadership bid.

“I must tell you, my friends, you who have waited faithfully for the punchline of this speech, that having consulted colleagues and in view of the circumstances in parliament, I have concluded that person cannot be me,” Johnson said at the news conference in a London luxury hotel.

Supporters in parliament, who had gathered expecting to hear him formally announce his candidacy, were left stunned.

Gove, a close friend of Cameron’s despite differences with the prime minister over Europe, had previously said he would back Johnson.

But in an article in the Spectator magazine on Thursday, Gove wrote that he had come “reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead”.

Conservative MPs 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.

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.

Johnson, known for a jokey public persona and a mop of unkempt blonde hair, became a popular national figure during eight years as London mayor, and used his charm to aid the Leave cause after deciding only late in the day to push for Brexit.

Several leading Conservatives questioned whether Johnson had the gravitas to run tough talks to mend the broken relationship with the EU and drive the country’s future on the global stage.

In an article in The Times newspaper, May took aim at Johnson’s persona by saying government was not “a game”.

She also appealed to the working classes, many of whom voted to leave the EU in protest at an elite who, they say, failed to cushion their lives from increasing competition.

One senior Conservative lawmaker, Crispin Blunt, said Gove had probably withdrawn his support because Johnson refused to promise him a job.

Britain’s new prime minister faces a huge task to unite the party and country, and persuade the EU to offer some kind of deal – balancing the desire expressed by voters to reduce immigration with London’s hope to maintain access to EU markets.

In the week since the referendum, Johnson had published a newspaper column promising curbs on immigration and continued access to the European common market, a position European officials say is untenable.

Conservative Party MPs will narrow a field of five leadership candidates down to two, and party members will then vote on which of them will become party leader and presumptive prime minister.

In addition to May and Gove, the candidates are Stephen Crabb, the cabinet minister responsible for pensions, Liam Fox, a right-wing former defence secretary, and Andrea Leadsom, a minister in the energy department.

Aware of the uncertainty in Britain, the party has said it is moving as quickly as it can to replace its leader and would do so by September 9.

The main opposition Labour Party also faces a potential leadership battle, with MPs having voted no confidence in left-wing party leader Jeremy Corbyn, who refuses to step down.

The vacuum at the top of both major political parties has added to the political uncertainty at a time when Britain faces its biggest constitutional change since the dissolution of its empire in the decades after World War II.

-Reuters