Boris Johnson to join campaign for UK to leave EU at referendum

The London Mayor is expected to confirm decision shortly

London Mayor Boris Johnson arrives back at his home (Reuters)

Boris Johnson will lend his support to the campaign for Britain to leave the EU at the referendum in June, The Independent understands.

The Mayor of London will officially announce which camp he will be behind when his column for the Daily Telegraph is released at 10pm on Sunday night.

But after listening to David Cameron put the case for remaining in the bloc on the Andew Marr Show, Mr Johnson has made up his mind and will campaign for "out", informed sources have said.

Those lobbying for Britain to leave the bloc have been courting the mayor for some time, hoping he will prove a popular figurehead alongside the likes of Michael Gove, Iain Duncan Smith and Nigel Farage.

It is a blow for the Prime Minister, who issued a personal plea to Mr Johnson to join him in supporting the re-negotiation deal agreed with Brussels.

"I would say to Boris what I say to everybody else, which is that we will be safer, we will be stronger, we will be better off inside the EU," the Mr Cameron told Andrew Marr.

"I think the prospect of linking arms with Nigel Farage and George Galloway and taking a leap into the dark is the wrong step for our country."

Mr Farage said earlier he expected Mr Johnson to join the Leave campaign, telling Sky News' Murnaghan show: "I think he will, and 'hurrah' is all I can say to that.

"What again I think a lot of the commentariat in Westminster don't understand is there are literally only five or six people in this referendum whose campaigning, whose presence, can sway the undecideds, and he is one of those half a dozen."

Speaking to the BBC on Sunday, Mr Duncan Smith said Britain's membership of the EU leaves the country vulnerable to a Paris-style terrorism attack.

The Work and Pensions Secretary, one of six ministers to declare for the "out" camp following Saturday's Cabinet meeting - said the UK's "open border" meant there was a lack of control over people entering the country.

But he faced criticism for "scaremongering" and lowering the tone of the debate just two days after Mr Cameron secured a deal on reforms if Britain stays in.

The pro-EU former deputy prime minister Lord Heseltine said: "I hope that Mr Duncan Smith's comments about terrorism are not typical of the scaremongering that could so easily characterise those arguing to leave Europe."