LONDON Prime Minister David Cameron urged rival Boris Johnson on Sunday not to join the campaign for a British exit from the European Union as the London mayor kept Britain guessing over a decision that could influence millions of voters in a June referendum.
Cameron called the June 23 poll after clinching a deal from other EU leaders that he said will give Britain special status in the world's biggest trading bloc, though six of his cabinet rebelled and are campaigning to leave the EU.
Johnson, a political showman whose buffoonish and eccentric exterior masks fierce ambition to succeed Cameron, has so far been silent, though British media has speculated that the 51-year-old will join the campaign to leave the EU.
By backing an EU exit, Johnson would transform the campaign and increase the prospects of Britain leaving the bloc because of his ability to swing public opinion. He was due to declare his position at 2200 GMT on Sunday.
Cameron cautioned Johnson, instantly recognisable by a thatch of platinum-blond hair, against joining opponents of the EU such as UK Independence Party chief Nigel Farage and maverick campaigner George Galloway.
"The prospect of linking arms with Nigel Farage and George Galloway and taking a leap into the dark is the wrong step for our country and 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," Cameron said.
"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.
Johnson has repeatedly stalled over staking out a clear position on Britain's EU membership, at times railing against a perceived overreach into national affairs, while at others praising its positive impact on London's role as a global trading centre.
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 from the EU 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.
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.
Though 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.
Opposed are several bickering 'out' campaign groups which failed to turn any of Cameron's most senior cabinet colleagues, though one close ally, Justice Secretary Michael Gove, did rebel with five other cabinet colleagues.
As one of Britain's most popular politicians, Boris could shift the current balance of the referendum campaign.
"Boris, from an out campaign point of view, would be a gain because he would slightly rebalance things," said Andrew Rawnsley, chief political commentator for the Observer.
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 must also calculate whether his chances of succeeding Cameron, who will step down before 2020, are increased by joining the 'out' campaign or supporting Cameron.
If he jumped the wrong way and was wrong-footed by a referendum decision his political chances of one day succeeding Cameron might be doomed.
"WE'D LOVE BORIS"
Farage, one of the 'out' campaign's leaders, made an appeal for Johnson to back the movement which has so far failed to find a unifying figurehead.
"He's one of those half a dozen people who reaches out to a large number of voters and we'd love to see 'Bo-Go' as the headline tomorrow," Farage said.
Farage, who casts the EU as a doomed German-dominated bloc that holds back Britain's $2.9 trillion economy and punches way below its weight on the world stage, dismissed criticism that he had stumbled by enlisting Galloway.
When Galloway, a firebrand former lawmaker who was expelled from the Labour Party for opposing its 2003 Iraq war, appeared on stage at an 'out' campaign rally on Friday night, some campaigners walked out.
Johnson has previously set out Britain's sovereignty over the EU as one of his key areas of concern, asking Cameron in parliament earlier this month to negotiate a deal that restricted Brussels' reach into national affairs.
Cameron said he would set out such proposals but declined to give details.
(Additional reporting by William Schomberg; Editing by Guy Faulconbridge and Richard Balmforth)
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