Andy Coulson has quit as Downing Street communications chief.
In a personal statement issued by Number 10, Mr Coulson said he was "proud" of his work with David Cameron and the coalition.
But he added: "Nothing is more important than the Government's task of getting this country back on its feet.
"Unfortunately continued coverage of events connected to my old job at the News of the World has made it difficult for me to give the 110% needed in this role.
"I stand by what I've said about those events but when the spokesman needs a spokesman it's time to move on.
"I'll leave within the next few weeks and will do so wishing the Prime Minister, his family and his brilliant and dedicated team the very best for what I'm sure will be a long and successful future in Government."
Labour MP Tom Watson, who has been pursuing the allegations against Mr Coulson and the News of the World, accused him of sneaking out the statement on a busy news day.
He also said the resignation raised "serious questions" about the Prime Minister's judgment.
"This is the second job that Andy Coulson has resigned from for something he claims to know nothing about," Mr Watson said.
"His departure creates serious questions over the Prime Minister's judgment and points to the need for a deeper investigation into the affairs of Rupert Murdoch's News Corp.
"It's a mark of the man that he would sneak out a statement on a Friday morning on a busy news day.
"Spin and obfuscation is all we get from Downing Street - we need to get to the truth."