Why Boris Johnson is dropping out of the race for British prime minister

The former London mayor's decision has shocked the public and much of the political scene.

Boris Johnson, the former London mayor who took a prominent role in the campaign for Britain to leave the European Union, announced on Thursday he would be dropping out of the race for prime minister, in a decision that upended newly settled expectations of the post-Brexit political scene.

In a speech on Thursday widely expected to contain a formal announcement of his candidacy, Mr. Johnson defended his record as mayor of London, delivered a riposte to US President Barack Obama’s comment that Britain would be at the "back of the queue" on trade if they left the European Union, and said the Conservatives should "speak up for those forgotten people" who voted for Britain to leave.

Then he dropped a bombshell.

"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," he said.

The statement elicited audible gasps from those present and an outpouring of disbelief from Twitter users. Other Conservative Party members expressed shock, with former Secretary of State for Defence and current candidate for prime minister Liam Fox saying he was "absolutely gobsmacked" by the news.

The decision may be linked to key ally and Justice Secretary Michael Gove's decision to withdraw his support for Johnson and announced that he would run for prime minister himself.

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

It also followed the leak of an email from Mr. Gove's wife, Daily Mail columnist Sarah Vine, in which Ms. Vine urged her husband and his aides to obtain specific guarantees on immigration policy from Johnson before making a deal to support him.

Johnson, an eccentric figure whose perpetually rumpled hair and clumsy demeanor, helped underscore the populist appeal to the "leave" campaign, had broken with current Prime Minister David Cameron over the issue and seemed to drop abundant hints that he aspired to Britain's top office.

Gove, conversely, had long disavowed any such ambition, even saying in 2012 that he would be willing to "sign a piece of parchment in my own blood saying I don't want to be prime minister."

The current favorite for the Conservative nomination is Home Secretary Theresa May, who backed remaining in the European Union but kept a low profile during the campaign. Many within the party see her as a potential unifier. And as home secretary, she introduced a series of strict measures on immigration, including minimum salary thresholds for non-EU workers who sought to move to Britain and another such requirement for British citizens who wanted to bring a spouse or a child to the country, according to Time.

The other candidates for the Conservative nomination include work and pensions secretary Stephen Crabb, energy minister Andrea Leadsom, and former Defense Secretary Fox.

This report contains material from the Associated Press and Reuters.