Boris Johnson has ignored Prime Minster David Cameron's last minute plea for him to join the campaign for Britain to stay in the European Union (EU). "After a huge amount of heartache" the mayor of London will join the campaign for Britain to leave the EU.

The Conservative MP and cabinet minister said his decision had been "agonising" but he felt the EU was eroding British sovereignty and David Cameron's latest reform deal would not bring about the changes he feels are needed for Britain to continue in the EU. His decision, following intense speculation about which side he would back, pits him against his party's leader who said leaving the EU would be a "leap into the dark" and "the wrong step for our country".

"If Boris and others really care about getting things done then the EU is one of the ways that we get things done," he told the BBC's Andrew Marr Show.

He also denied that defying the PM's wishes was his pitch to replace David Cameron as Conservative Party leader and eventually become prime minister.

Cameron's deal is bad

Praising Cameron for making the deal "he pulled off in a very short space of time", he said that he had told David Cameron that whatever happens at the end of this "he's got to stay".

"I did not want to do anything, 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, adding that he would not appear in "loads of TV debates against other members of my party."

But he added: "I don't think anybody could realistically claim that this is fundamental reform of the EU or of Britain's relationship with the EU."

He said that the trouble with Europe was that it was eroding British sovereignty "and you are seeing it more and more of employment, over border controls, over human rights, over all sorts of stuff".

UK not longer independent

"You have got a supreme judicial body in the European Court of Justice that projects down on this entire 500m people territory a single unified judicial order from which there is absolutely no recourse and no comeback and in my view that has been getting out of control," he added. "There is too much judicial activism, there is too much legislation coming from the EU."

He said that those in the against camp were "portrayed a crazy cracks and all the rest of it", adding that he didn't mind because he happened to think that he was right."

"I've thought about it for many many years and I don't see how having worried about this issue for quite so long, 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 added.