Conservative Mayor of London Boris Johnson has said he has decided “after a huge amount of heartache” to campaign for Britain to leave the EU.
He said the EU was eroding British sovereignty and PM David Cameron’s reform deal would not bring about the fundamental change that was needed.
His decision pits him against Mr Cameron, who says Britain will be “safer and stronger” within the EU.
The prime minister says leaving the EU would be a “leap in the dark”.
The announcement by Mr Johnson, who many believe has ambitions to lead the Conservative party, follows intense 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.
She said Mr Johnson, MP for Uxbridge and South Ruislip, informed Mr Cameron of his decision by text message shortly before making it public.
Addressing reporters outside his home in north London, Mr Johnson said the EU was a “political project” that was in “real danger of getting out of proper democratic control”.
He said UK sovereignty – the power of Britain to govern itself – was being “very greatly eroded” by EU institutions, with “too much judicial activism” and legislation coming out of the EU.
Analysis, BBC deputy political editor James Landale
Boris Johnson is political box office. He is one of Britain’s most charismatic politicians. So his decision to support Brexit gives popular appeal to a Leave campaign that has lacked a talismanic figurehead.
And polls suggest that his is a voice that many will listen to as they make up their minds. So the Mayor of London will transform the referendum campaign. But will he transform the result?
Until now Mr Johnson has won his votes as a jovial character in two regional elections. He is now testing his appeal on the national stage in a contest of historic importance. And that matters because Mr Johnson could be our prime minister one day.