British PM Squares Up for EU Referendum Fight

The Conservative leader will present to parliament the reforms secured at an EU summit in Brussels last week which he argues give Britain "special status" and are enough reason to vote to stay in the 28-member bloc in a June 23 referendum.

But he is facing a major challenge in Johnson, a Conservative rival who manages to reach across the political divide and is tipped as a future prime minister.

Johnson electrified the political landscape Sunday by throwing his weight behind the campaign for a so-called "Brexit", which until then had been marred by in-fighting and a lack of leadership.

Five cabinet ministers have already declared themselves in favor of the "Leave" campaign and reports suggest around a third of Cameron's 330 lawmakers could back a "Brexit", AFP reported.

Britain, which first joined the then European Economic Community (EEC) in 1973, has long had strained relations with the bloc, opting out of key projects including the euro and the Schengen passport-free zone.

Any departure would pose a major headache for the European Union, already grappling with its biggest migration crisis since World War II.

The June referendum will be Britain's second on European membership in just over 30 years after voters in 1975 backed membership of the then EEC by just over 67 percent.

Johnson's intervention adds credibility to the "Leave" campaign, which includes figures as diverse as anti-immigration UK Independent Party (UKIP) leader Nigel Farage and maverick far-left former lawmaker George Galloway.

Cameron has defended his Brussels renegotiation by warning the country would lose power if it left the EU.

"Yes, of course if Britain were to leave the EU that might give you a feeling of sovereignty but you've got to ask yourself 'is it real?'," he said.