London Mayor Boris Johnson has revealed he will campaign for Britain to leave the EU in a major blow to the British Prime Minister David Cameron.

The decision by Mr Johnson ends months of speculation over which side he would support and represents a huge coup for the 'Leave' campaign, given his broad appeal among voters.

Mr Johnson said he came to his decision after a "great deal of heartache" but that he wanted "a better deal for the people of this country".

While he said Mr Cameron had done "fantastically well" in the deal he reached with Brussels he added no one "could realistically claim that this is fundamental reform of the EU".

Mr Johnson said: "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's anything else I could do".

"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".

Image: Dominic Lipinski / PA Wire/Press Association Images

He added: "What I won't do is take part in loads of TV debates against other members of my party and I was told what the Prime Minister had to said this morning about not sharing platforms with George Galloway and other individuals and I won't do that either".

He also rejected his move was connected to his leadership ambitions.

"Whatever happens at the end of this, and I have said this to the Prime Minister, he has got to stay," Mr Johnson said.

Mr Cameron had made a last-ditch plea for the Conservative MP to campaign for the UK to remain part of the 28-member bloc.

UKIP leader Nigel Farage said he was "delighted" at Mr Johnson's backing for the Leave campaign.

Mr Johnson joins six other ministers who attend cabinet in backing the campaign to leave after Mr Cameron called a referendum for June 23rd.