Brexit Campaigner Boris Johnson Says He Won't Run for Prime Minister

LONDON— -- The battle for the United Kingdom's Conservative Party leadership has been transformed after former London Mayor Boris Johnson, a vocal proponent of Britain's exiting the European Union, unexpectedly announced that he will not run for prime minister.

"My role will be to give every possible support to the next Conservative administration to make sure that we properly fulfill the mandate of the people that was delivered at the referendum," Johnson said in a speech today, referring to the U.K.'s June 23 vote to leave the EU, "and to champion the agenda that I believe in, to stick up for the forgotten people of this country."

Johnson was at the forefront of the pro-Brexit side and had been widely expected to run for the party leadership.

Politicians Theresa May, Michael Gove, Stephen Crabb, Liam Fox and Andrea Leadsom have confirmed that they are running for the spot, hoping to succeed Prime Minister David Cameron, who, after his failed efforts to keep the U.K. in the EU, said he will step down later this year.

Justice Secretary and Lord Chancellor Gove had been widely expected to be Johnson's running mate, but many people suspect he feared Johnson would backtrack on the Brexit. Gove announced on Wednesday that he was running for the leadership, saying he had come "to the conclusion that Boris cannot provide the leadership or build the team for the task ahead.”

The other top contender is Home Secretary May, who backed staying in the EU. She presented herself in a speech today as an experienced politician who is "ready and able to do the job from Day One.”

"If you are from an ordinary working-class family, life is just much harder than many people in politics realize," May said, developing her vision for the country. "Frankly, not everybody in Westminster understands what it's like to live like this, and some need to be told that it isn't a game. It's a serious business that has real consequences for people's lives."

When Cameron announced his intention to step down, many speculated that a general election would be organized after the new Conservative leader was chosen.

But May said today that if she is chosen as the party leader, there would be no general election until 2020.

Meanwhile, Work and Pensions Secretary Crabb, Energy Minister Leadsom and former Cabinet minister Fox are also contenders.

The U.K.’s main political parties — Labour and Conservative — have been in turmoil since the vote to leave the bloc. Jeremy Corbyn, the leader of the Labour Party, today refused to step down, despite a majority of his party’s ministers saying they have no confidence in him.