Boris Johnson announces he will campaign for Brexit in referendum

Boris Johnson has declared that he is to join the campaign for Britain to leave the European Union.

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.