London Mayor Boris Johnson Backs Brexit in Blow for Cameron

London mayor Boris Johnson on Sunday said he would support a vote for Britain to leave the European Union in a blow for Prime Minister David Cameron ahead of a membership referendum in June.

"After a great deal of heartache... I will be advocating vote Leave," said Johnson, a popular politician from Cameron's own Conservative Party who is seen as a potential successor to the prime minister.

His comments will be seen as a key victory by supporters of Britain's departure from the EU -- or "Brexit" -- although Johnson in his comments implied he would not lead the movement and would not take part in frontline campaigning.

He said Cameron had done "fantastically well" on negotiating reforms at an EU summit last week but 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."

"There should be no confusion between the wonders of Europe and the holidays in Europe and fantastic food and friendships... with a political project that has basically been going on now for decades," he said.

He said the European Court of Justice's power "has been getting out of control" and called for "a new relationship" with the EU based on trade and cooperation.

Johnson was speaking outside his home in Islington in north London after Cameron in an interview earlier on Sunday urged his party ally to join the camp of those supporting Britain's EU membership.