BRUSSELS: The European Commission said on Monday it would not take part in the campaign to keep Britain in the EU ahead of an in-out referendum in June.

“The Commission will not campaign, will not take part in the campaign,” spokesman Margaritis Schinas told a news briefing.

“Our role of facilitator between the British government and its 27 partners stops here.”

British Prime Minister David Cameron persuaded lawmakers that Britain's future lies in the European Union, as he squares up for a referendum fight against charismatic London mayor Boris Johnson.

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 on 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.

"Cameron's worst fear," the right-wing Daily Telegraph said of Johnson's declaration, while the left-leaning Daily Mirror said it was a "Dagger in Cam's heart".

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

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.

Defence Secretary Michael Fallon sought to play down the impact of Johnson's defection.