Boris Johnson throws his weight behind Brexit

The London mayor put an end to months of speculation, saying that David Cameron’s re-negotiation had failed to deliver fundamental change in Britain’s relationship with Brussels.

“I don’t think that anybody can claim that this is fundamental reform of the EU or of Britain’s relationship with the EU,” he said.

His announcement - made outside his London home - is a huge boost for the “out” campaign potentially giving them a popular figurehead able to connect with voters in a way few other Westminster politicians can manage.

At the same time, it comes as a bitter blow for David Cameron who had long believed that his old rival from their days at Eton and Oxford would ultimately fall in behind his EU re-negotiation package.

Amid chaotic scenes, Mr Johnson insisted that he had agonised over the decision before finally declaring his hand.

“The last thing I wanted was to go against David Cameron or the Government but after a great deal of heartache I don’t think there is anything else I can do,” he said.

“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.”

Earlier, Mr Cameron had issued a last ditch appeal for the London mayor not to align himself with “outers” like Ukip leader Nigel Farage and Respect’s George Galloway.

“I think 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,” he said.

“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.”

Mr Johnson said that he did not intend to take part in “loads” of debates with other members of his party and insisted he would not share a platform with people like Mr Galloway.

He strongly denied claims that he was positioning himself for a leadership bid with an appeal to the Tory grassroots who are fiercely Eurosceptic.

“Whatever happens at the end of this - and I’ve said this to the Prime Minister - he’s got to stay,” he said.

Critics, however, were quick to accuse him of putting his own ambition ahead of the national interest, pointing out that while he had long been critical of the EU he had no track record as an “outer”.

Liberal Democrat leader Tim Farron said Mr Johnson had had “more positions on Europe than the Kama Sutra”.

“This is a deeply cynical move from a deeply ambitious politician who is using an in-out referendum as a back door to Number 10. It is a selfish move to put personal ambition before the jobs, security and prosperity of every Londoner,” he said.

Shadow foreign secretary Hilary Benn said: “Boris Johnson in the past has written a lot about the importance of staying in the European Union and if he is actually thinking about putting his personal leadership ambitions above the national interest I don’t think it’s going to do him any good.”

Mr Johnson’s announcement completely overshadowed Mr Cameron’s efforts to make the case for his renegotiation deal hammered out in marathon talks in Brussels.

Earlier, the Prime Minister dismissed claims that Brexit would restore sovereignty to the UK, saying in practice Britain was able to exercise far greater leverage internationally from within the EU.

“If Britain were to leave the EU that might give you a feeling of sovereignty but you have got to ask yourself ‘is it real?’,” he said.

“You have an illusion of sovereignty but you don’t have power, you don’t have control, you can’t get things done.”