Boris Johnson has galvanised the EU referendum campaign with a declaration that he is to join the campaign for Britain to leave.
A day after David Cameron appealed on the steps of Downing Street for voters to back his EU re-negotiation deal, the London mayor said it 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 - putting an end to months of speculation - is a huge boost for the Brexit 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 Mr 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 outside his north London home, Mr Johnson said that he had agonised over the decision.
"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."
The announcement drew a furious response from the Conservative former deputy prime minister Lord Heseltine who warned that a vote to leave the EU would be devastating for the City of London.
"If he were to be successful in his ambition to cut us off from Europe, the flags would fly in Frankfurt and Paris in his honour," he said.
"At a stroke, he would have blown away the safeguards for our financial services industry that the Prime Minister has just secured."
Critics were swift to claim that the real reason for Mr Johnson's announcement was to position himself for a potential challenge for the Tory leadership, saying that he had no track record as an "outer".
Earlier, Mr Cameron - who was reportedly only notified of Mr Johnson's decision in a text sent a few minutes before he made the announcement - had issued a last ditch appeal for him not to align himself with the "out" campaign.
"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 told BBC One's The Andrew Marr Show.
"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."
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."
In contrast, Mr Johnson said that action was needed because the European "political project" was "in danger of getting out of proper democratic control".
"Sovereignty is people's ability - the ability of the public - to control lives and to make sure that the people they elect are able to pass the laws that matter to them. The trouble is, with Europe that is being very greatly eroded," he said.
Opposition parties said he was putting personal ambition ahead of the national interest. Liberal Democrat leader Tim Farron said the mayor 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."