LONDON — (CNN) 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.”

“I will be advocating Vote Leave … 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.

Johnson’s 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 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.

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.

Britons will vote in an “in or out” referendum on June 23. Cameron argues the British economy will suffer if the country opts out.

In a 2,000-word opinion piece in the Telegraph on Sunday, Johnson said leaving was a “once-in-a-lifetime chance to vote for real change.”

“This is a moment to be brave, to reach out — not to hug the skirts of Nurse in Brussels, and refer all decisions to someone else,” he wrote.

Before Johnson showed his hand, Cameron appeared on Sunday morning television to push for a vote to stay.

“If Boris and if others really care about being able to get things done in our world, then the EU is one of the ways in which we get them done,” he said.

“Having that seat at the table in the EU — just as being a member of NATO — is a vital way that we project our values and our power and our influence in the world.”

Britain is one of the biggest economies in the European Union — and is a net contributor to the EU budget.

Critics say membership of the EU is a costly burden that brings regulations and excessive migration. Advocates say membership is good for the economy and leaving could be an expensive disaster.

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