With 122 days to go until the EU in/out referendum on 23rd June, the OUTers have been given a massive boost today with London Mayor Boris Johnson joining their ranks.
Johnson, who is also the Conservative MP for Uxbridge and South Ruislip in west London, is expected to confirm his decision in a statement later and set out his reasons in a Telegraph column.
BBC political editor Laura Kuenssberg said Mr Johnson's decision would be seen as a huge boost to the Out campaign and a major blow to the prime minister, who had hoped to persuade friends and rivals to back the campaign to remain.
Mr Cameron issued a last-ditch plea to Mr Johnson - seen as a potential future Conservative Party leader - to back staying in the EU, during an appearance on the BBC's Andrew Marr Show earlier.
Other big hitters in the Leave campaign are George Galloway, the former Respect MP for Bradford West and, of course, Ukip leader Nigel Farage.
Boris Johnson to campaign to leave the EU
BBC News
21 February 2016
Conservative Mayor of London Boris Johnson is to campaign for Britain to leave the EU, the BBC understands.
The MP's decision comes as ministers began campaigning over the UK's EU membership, after PM David Cameron called a referendum for 23 June.
Mr Johnson is expected to confirm his decision in a statement later and set out his reasons in a Telegraph column.
Mr Cameron - who is recommending the UK remains in the EU - told the BBC leaving would be "a leap in the dark".
A statement from Mr Johnson, the MP for Uxbridge and South Ruislip, is expected at about 17:00 GMT, following days of speculation about which side he would back.
BBC political editor Laura Kuenssberg said Mr Johnson's decision would be seen as a huge boost to the Out campaign and a major blow to the prime minister, who had hoped to persuade friends and rivals to back the campaign to remain.
Mr Cameron issued a last-ditch plea to Mr Johnson - seen as a potential future Conservative Party leader - to back staying in the EU, during an appearance on the BBC's Andrew Marr Show earlier.
'Safer and stronger'
Respect's George Galloway (left) and Ukip's Nigel Farage, who are both at the opposite ends of the political spectrum, make for unlikely bedfellows within the Leave campaign
He said if Mr Johnson cared about "getting things done" in the world, being a member of the EU was key.
And, he added, "linking arms" with Nigel Farage and George Galloway - part of the Grassroots Out movement which is one of two groups vying to become the official 'Leave' campaign - is "taking a leap into the dark and is the wrong step for our country".
Mr Cameron, who renegotiated the terms of Britain's membership of the EU, argues that the UK is "better, safer and stronger" being in the EU.
And he said that withdrawing from the 28-member bloc might give the "illusion of sovereignty" but would actually weaken the UK's power and influence.
However, his argument for remaining in the union was not enough to convince Mr Johnson, who joins six other ministers who attend cabinet in backing the campaign to leave.
Among them are Work and Pensions Secretary Iain Duncan Smith and Justice Secretary Michael Gove.
Zac Goldsmith, who is hoping to become Mr Johnson's successor as London Mayor in May, has also said he will vote to leave.
Some of those backing Brexit had spoken of their hopes that Mr Johnson would become the figurehead of the Out campaign, and reach out to large swathes of voters in a way that many politicians in Westminster cannot.
Boris Johnson says UK is better off outside the EU - BBC News
Johnson, who is also the Conservative MP for Uxbridge and South Ruislip in west London, is expected to confirm his decision in a statement later and set out his reasons in a Telegraph column.
BBC political editor Laura Kuenssberg said Mr Johnson's decision would be seen as a huge boost to the Out campaign and a major blow to the prime minister, who had hoped to persuade friends and rivals to back the campaign to remain.
Mr Cameron issued a last-ditch plea to Mr Johnson - seen as a potential future Conservative Party leader - to back staying in the EU, during an appearance on the BBC's Andrew Marr Show earlier.
Other big hitters in the Leave campaign are George Galloway, the former Respect MP for Bradford West and, of course, Ukip leader Nigel Farage.
Boris Johnson to campaign to leave the EU
BBC News
21 February 2016

Conservative Mayor of London Boris Johnson is to campaign for Britain to leave the EU, the BBC understands.
The MP's decision comes as ministers began campaigning over the UK's EU membership, after PM David Cameron called a referendum for 23 June.
Mr Johnson is expected to confirm his decision in a statement later and set out his reasons in a Telegraph column.
Mr Cameron - who is recommending the UK remains in the EU - told the BBC leaving would be "a leap in the dark".
A statement from Mr Johnson, the MP for Uxbridge and South Ruislip, is expected at about 17:00 GMT, following days of speculation about which side he would back.
BBC political editor Laura Kuenssberg said Mr Johnson's decision would be seen as a huge boost to the Out campaign and a major blow to the prime minister, who had hoped to persuade friends and rivals to back the campaign to remain.
Mr Cameron issued a last-ditch plea to Mr Johnson - seen as a potential future Conservative Party leader - to back staying in the EU, during an appearance on the BBC's Andrew Marr Show earlier.
'Safer and stronger'

Respect's George Galloway (left) and Ukip's Nigel Farage, who are both at the opposite ends of the political spectrum, make for unlikely bedfellows within the Leave campaign
He said if Mr Johnson cared about "getting things done" in the world, being a member of the EU was key.
And, he added, "linking arms" with Nigel Farage and George Galloway - part of the Grassroots Out movement which is one of two groups vying to become the official 'Leave' campaign - is "taking a leap into the dark and is the wrong step for our country".
Mr Cameron, who renegotiated the terms of Britain's membership of the EU, argues that the UK is "better, safer and stronger" being in the EU.
And he said that withdrawing from the 28-member bloc might give the "illusion of sovereignty" but would actually weaken the UK's power and influence.
However, his argument for remaining in the union was not enough to convince Mr Johnson, who joins six other ministers who attend cabinet in backing the campaign to leave.
Among them are Work and Pensions Secretary Iain Duncan Smith and Justice Secretary Michael Gove.
Zac Goldsmith, who is hoping to become Mr Johnson's successor as London Mayor in May, has also said he will vote to leave.
Some of those backing Brexit had spoken of their hopes that Mr Johnson would become the figurehead of the Out campaign, and reach out to large swathes of voters in a way that many politicians in Westminster cannot.
Boris Johnson says UK is better off outside the EU - BBC News
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