London mayor Boris Johnson on Sunday said he would support a vote for Britain to leave the European Union in a blow for Prime Minister David Cameron ahead of a membership referendum in June.
"After a great deal of heartache... I will be advocating vote Leave," said Johnson, a popular politician from Cameron's own Conservative Party who is seen as a potential successor to the prime minister.
His comments will be seen as a key victory by supporters of Britain's departure from the EU - or "Brexit" - although Johnson in his comments implied he would not lead the movement and would not take part in frontline campaigning.
He said Cameron had done "fantastically well" on negotiating reforms at an EU summit last week but added: "I don't think anybody could realistically claim that this is fundamental reform of the EU or of Britain's relationship with the EU".
"There should be no confusion between the wonders of Europe and the holidays in Europe and fantastic food and friendships... with a political project that has basically been going on now for decades," he said.
He said the European Court of Justice's power "has been getting out of control" and called for "a new relationship" with the EU based on trade and cooperation.
Johnson was speaking outside his home in Islington in north London after Cameron in an interview earlier on Sunday urged his party ally to join the camp of those supporting Britain's EU membership.
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.
'Bo-go Boris'?
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, Johnson 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 $2.9tn 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.
