British Prime Minister David Cameron's decision to resign after losing the referendum vote on European Union membership will set off an intense Conservative Party leadership battle.

Cameron said Friday a new prime minister should be in place by a party conference in October.

Among the possible contenders are former London Mayor Boris Johnson and Justice Secretary Michael Gove, who both helped lead the "leave" campaign, and Home Secretary Theresa May.

Other Cabinet members are likely to contend as well. Treasury chief George Osborne's chances seem damaged by the Brexit vote as he had argued strongly to remain in the EU.

Conservative Party rules call for the party's members of Parliament to choose two candidates through a series of ballots, and then the entire party's membership will choose between those two.

If only one candidate surfaces by the close of nominations, he or she is declared the leader and the choice then must be ratified by the membership.

Johnson is expected to make a statement Friday morning after the victory of his "leave" campaign.

Defense Secretary Michael Fallon said it is too early to speculate about who will replace Cameron, who became party leader in 2005 and led the party to two general election victories.

"I think it is a bit too early to start speculating about that," he said. "There is plenty to do now to help make this decision work, to stabilize our economy, to reassure our allies and to continue the program we were all elected on last year."

He said Cameron has done the honorable thing by resigning.

Cameron had been expected to serve several more years. He was re-elected last year with a majority vote but lost control of the debate on the EU.

With his wife Samantha at his side, he said he is not the correct "captain" to steer Britain through its upcoming negotiations.