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."
What has the EU ever done for us?
What has the EU ever done for us?
1/7 1. It gives you freedom to live, work and retire anywhere in Europe
As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states.
2/7 2. It sustains millions of jobs
A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU.
3/7 3. Your holiday is much easier - and safer
Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed.
4/7 4. It means you're less likely to get ripped off
Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe.
5/7 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime
Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime.
6/7 6. Our businesses depend on it
According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business.
7/7 7. We have greater influence
Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.
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."