LONDON Charismatic London Mayor Boris Johnson was set on Sunday to lend his pulling power to the campaign for a British exit from the European Union, dealing a heavy blow to Prime Minister David Cameron ahead of June's referendum.
The BBC reported Johnson's decision without citing sources ahead of an official announcement expected around 5 p.m. (1700 GMT). One source with knowledge of the decision confirmed to Reuters that he would back the 'out' campaign.
The decision comes a day after Cameron called the June 23 poll and declared his own intention to campaign for Britain to stay in the EU after clinching concessions from other member states that he said would give Britain a special status in the world's biggest trading bloc.
Johnson, a political showman whose buffoonish and eccentric exterior masks fierce ambition to succeed Cameron, has the potential to galvanise the 'out' campaign because of his ability to swing public opinion.
Cameron on Sunday urged Johnson, instantly recognisable by his thatch of platinum-blond hair, not to join opponents of EU membership, loosely organised around divisive political figures such as UK Independence Party chief Nigel Farage and maverick left-wing campaigner George Galloway.
"I would say to Boris what I say to everybody else, which is that we will be safer, we will be stronger, we will be better off inside the EU," Cameron told the BBC.
It was not only Cameron who had been waiting to hear Johnson's decision.
Leaders on both sides of the Brexit debate, Britain's Sunday newspapers and even financial markets had been eagerly waiting for Johnson to declare his allegiance. Johnson joins six members of Cameron's cabinet who on Saturday rebelled and said they wanted to leave.
British voters - and Cameron's ruling Conservative Party - are split on membership. Polls suggest about a fifth of voters are undecided, though betting odds have moved further in favour of Britain remaining and a 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.
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 is 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 has also had to calculate whether his chances of succeeding Cameron, who will step down before 2020, are increased by joining the 'out' campaign.
If the referendum shows that he has jumped the wrong way and been wrong-footed by the referendum decision, his political chances of one day succeeding Cameron might be doomed.
(Additional reporting by William Schomberg and Jemima Kelly; Editing by Guy Faulconbridge, Richard Balmforth and Digby Lidstone)
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