London: London Mayor Boris Johnson said on Sunday he would back Britain's exit from the European Union in a June 23 referendum, ending intense speculation over the position of one of British politics' most influential figures.
Speaking to a crowd of reporters on the steps of his London home, Johnson said Prime Minister David Cameron had not achieved fundamental reform of the European Union.
"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 said.
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 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.
Earlier, in the day, Cameron urged rival Boris Johnson not to join the campaign for a British exit from the European Union.
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.
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.
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.