British Pound falls after popular Mayor of London backs 'Vote Leave' campaign

British Prime Minister David Cameron's campaign to keep the UK in the European Union appears to have met its stiffest opposition, with London's colorful, popular mayor, Boris Johnson, entering the debate on the side of the "Vote Leave" campaign Sunday.

Johnson announced his support for leaving the EU before a huge crowd of reporters Sunday afternoon, saying his decision to defy Cameron's position caused "a huge amount of heartache."

His intervention in the campaign appeared to have an immediate impact on the British pound when markets opened Monday, with the pound falling against all major currencies.

Cameron is expected to address the House of Commons, the lower house of Parliament, on Monday, the first day of the formal referendum campaigning period

At a news conference on Sunday, Johnson laid out his position. "I will be advocating Vote Leave," he told reporters, "because I want a better deal for the people of this country, to save them money and to take back control."

His support will give the campaign to leave the EU a charismatic figurehead -- and will pit two of Britain's most powerful, influential politicians against each other as they attempt to persuade Britons of the merits of their positions ahead of an "in or out" referendum on June 23.

The men, both members of the ruling Conservative party, are former pupils of elite private school Eton and belonged to the same exclusive "dining club" during their days at Oxford University.

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Cameron has been pushing for Britain to stay in the EU, and only last week struck a deal with the bloc's leaders to give the UK "special status."

But high-ranking members of his Cabinet, including Justice Secretary Michael Gove, Culture Secretary John Whittingdale and Work and Pensions Secretary Iain Duncan Smith, have all come out in support of the "Vote Leave" campaign.