LONDON (REUTERS) - Prime Minister David Cameron's ex-media chief was accused on Thursday of pure hypocrisy when, as editor of a Rupert Murdoch tabloid, he exposed a government minister's affair while conducting his own extra-marital relationship.

Appearing in the witness box for the sixth day at the tabloid phone-hacking trial at London's Old Bailey court, Andy Coulson said he regretted revealing in 2004 that then interior minister David Blunkett was having an affair.

Mr Coulson, who denies conspiracy to hack into mobile phone voicemails, had told the jury last week that a reporter on the now defunct News of the World had once played him excerpts of voicemail messages left by Mr Blunkett on the mobile phone of a woman with whom he was having a relationship.

That enabled Mr Coulson to confront Mr Blunkett about the relationship and splash the story on the paper's front page.

Mr Coulson has also told the trial he was engaged in an on-off relationship with his co-defendant Rebekah Brooks, another former Murdoch editor who went on to run the British newspaper arm of News Corp. "This story was about someone's private life and, given what is going on in my own private life, the irony is not lost on me," the 46-year-old told the court. "Pure hypocrisy isn't it?" he was asked by David Spens, the lawyer for another co-defendant, Clive Goodman.

After a pause Mr Coulson replied: "The irony is not lost on me."

Mr Coulson left the mass-selling Sunday tabloid in 2007 when Goodman, his royal editor, admitted hacking into the phones of royal aides. Mr Coulson said he had not known about the practice but had left to take full responsibility for the crime.

Facing questions from Mr Spens, Mr Coulson said he had not asked the reporter who showed him the hacked messages from Blunkett where they had come from, saying that he had just told the reporter to stop. "You are a very bright, intelligent man with a questioning mind," Mr Spens said to him about his failure to ask questions.

Mr Coulson replied: "I'm not telling the jury I wasn't curious, I'm telling the jury what happened. The most important thing in my mind was I stopped it."

Mr Coulson and Goodman also deny charges of making illegal payments to police officers. Brooks has also pleaded not guilty to charges over phone-hacking and other offences. The trial, which has now run for more than 100 days, continues.