The Prime Minister told the media: "I take full responsibility for employing Andy Coulson.
"I did so on the basis of undertakings I was given by him about phone hacking and those turned out not to be the case. I always said that if they turned out to be wrong, I would make a full and frank apology and I do that today.
"I am extremely sorry that I employed him. It was the wrong decision and I am very clear about that."
Cameron added: "I would say that no one has made any complaints about the work that he did for me either as leader of the opposition or indeed here in Number 10 Downing Street, but knowing what I now know and knowing that those assurances weren’t right it was obviously wrong to employ him. I gave someone a second chance and it turned out to be a bad decision."
Cameron appointed Coulson as his director of comms in 2007 and took him into Downing Street in 2010. Coulson resigned in January 2011 over the phone hacking scandal, before being arrested in July that year.
At the time, Cameron told the House of Commons that if it turned out Coulson had lied to him it would be the moment for a "profound apology".