Ex-London mayor halts bid to be UK prime minister, upends race

By Kylie MacLellan and Elizabeth Piper LONDON (Reuters) - Former London mayor Boris Johnson abruptly pulled out of the race to become Britain's prime minister that he was once favored to win, upending the contest less than a week after he led a campaign to take the country out of the EU. Johnson's announcement, to audible gasps from a roomful of journalists and supporters on Thursday, was the biggest political surprise since Prime Minister David Cameron quit after losing last week's referendum on British membership of the bloc. It makes interior minister Theresa May, a party stalwart who backed remaining in the European Union, the new favorite to succeed Cameron.