Leading Brexiter Boris Johnson says he won't run for prime minister

Boris Johnson: I will not stand for prime minister 01:13

London (CNN)The race to become Britain's next prime minister took a

surprise twist Thursday as leading Brexit campaigner Boris Johnson, considered a frontrunner to replace the outgoing David Cameron, announced that he would not be running.

Having outlined the demands of the role to a room full of journalists in London, Johnson announced: "I have concluded that that person cannot be me."

The Conservative MP and former London mayor was a prominent voice in the campaign to lead Britain out of the European Union -- an endeavor many saw as partly an effort to position himself as the future leader of the ruling Conservative Party, and of the country.

His decision not to run means that five Conservative MPs will compete to replace Cameron, who announced his intention to resign after losing his campaign to persuade voters to remain in the EU in a national referendum last week.

They include three Cabinet ministers: Home Secretary Theresa May, Justice Secretary Michael Gove and Work and Pensions Secretary Stephen Crabb. Conservative MPs Liam Fox and Andrea Leadsom are also running.

Conservative MPs who wished to contest the leadership had until noon local time (7 a.m. ET) to declare their run.