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 announcement by Mr Johnson, who many believe has ambitions to lead the Conservative party, follows intense speculation about which side he would back.