LONDON: London’s Mayor Boris Johnson on Sunday threw his weight behind the campaign for Britain to leave the European Union, in a blow for his old friend and rival Prime Minister David Cameron, who had appealed for his backing.

“After a great deal of heartache... I will be advocating vote ‘Leave’,” Johnson said, making his long-awaited announcement to a large crowd gathered outside his home in north London.

Johnson, a popular politician from Cameron’s Conservative Party who is seen as a potential successor to the British prime minister, said he wanted ties with Europe based on “trade and cooperation” and not “a political project.”

He said Cameron had done “fantastically well” in negotiating concessions from European Union leaders at a summit last week that paves the way for a membership referendum on June 23.

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.”

The declaration will be seen as a key victory for supporters of a British departure from the EU — or “Brexit.”

Compared to other anti-EU politicians, the mayor of London is popular even with those who do not share his political views, thanks to his witty soundbites and shambolic appearance.

“I’m really disappointed that he’s backing leave because I think we should stay,” said Andy Burton, 36, a director at a health care company who watched Johnson make his statement alongside around 70 other passers-by.

“He’s just got a real presence, he’s very popular, he will sway Londoners.”

The Leave.EU campaign, one of the groups advocating Brexit, welcomed Johnson’s endorsement.

Co-chairman John Tice said: “We share his vision of a UK with full, democratic control of its affairs.”