LONDON (REUTERS) - London Mayor Boris Johnson said he backed leaving the European Union, dealing a blow to Prime Minister David Cameron by giving the "out" campaign a de facto leader who is one of the most charismatic politicians in British politics.

Forty-three years after Britain joined the EU's predecessor, Cameron announced a June 23 referendum on Saturday after clinching a deal from 27 other leaders to give Britain what he said was a special status.

But Boris, one of Britain's most high-profile politicians, remained silent on Cameron's deal for nearly two days before making his dramatic announcement on the steps of his north London home.

"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," Johnson told a crowd of reporters.

The move deepened a divide in Cameron's ruling Conservative Party and betting odds against a British exit shortened to 2/1 (a 33 per cent chance) from 12/5 (29 per cent).

Johnson, a political showman whose buffoonish and eccentric exterior masks fierce ambition to succeed Cameron, dismissed questions about whether the announcement was a bid for the future leadership of the Conservative Party.

Johnson said the decision to oppose Cameron had been"agonisingly difficult" and praised his renegotiation effort, but that ultimately the reforms agreed in Brussels had fallen far short of meaningful change on issues like sovereignty.

"I don't see how, having worried about this issue for quite so long and having fulminated for quite so long about the lack of democracy in the EU, I can then pass up what I think will be the only chance any of us have in our lifetimes to put an alternative point of view," he said.

British voters - and Cameron's ruling Conservative Party - are split on membership. Polls suggest about a fifth of voters are undecided, though one poll published on Sunday showed the"in" campaign with a lead of 15 percentage points.

In what has been cast as Britain's biggest strategic decision in at least a generation, voters will be asked on June 23: "Should the United Kingdom remain a member of the European Union or leave the European Union?"

A British exit would rock the Union - already shaken by differences over migration and the future of the euro zone - by ripping away its second-largest economy, one of its top two military powers and by far its richest financial centre.

FIGUREHEAD

Pro-Europeans, including former prime ministers Tony Blair and John Major, have warned that an exit could also trigger the break-up of the United Kingdom by prompting another Scottish independence vote if England pulled Scotland out of the EU.

Though he is juggling a deeply divided party, Cameron's backing for EU membership has the support of the City of London, major companies, much of the Labour Party, major trade unions, international allies and even Scottish nationalists.

Johnson's popularity could galvanise those opposed to Cameron's position, providing a popular and recognisable figurehead for the several bickering "out" campaign groups which have so far failed to come up with a unified vision of Britain outside Europe.

An Ipsos MORI poll showed Johnson second only to Cameron when it comes to swaying public opinion on Europe. One in three voters said Johnson would be important in helping them decide which way to vote, the poll showed.

But in declaring his position on the most divisive issue in British politics, Johnson must also calculate whether his chances of succeeding Cameron, who will step down before 2020, are increased by joining the "out" campaign.