David Cameron's communications director, Andy Coulson, will come under fresh pressure to defend his editorship of the News of the World and his knowledge about the illegal activities of his journalists amid new allegations about the paper's involvement with private detectives who broke the law.

The Guardian has learned that while Coulson was still editor of the tabloid, the newspaper employed a freelance private investigator even though he had been accused of corrupting police officers and had just been released from a seven-year prison sentence for blackmail.

The private eye was well known to the News of the World, having worked for the paper for several years before he was jailed, when Coulson was deputy editor. He was rehired when he was freed.

Evidence seen by the Guardian shows that Mr A, who cannot be named for legal reasons, was blagging bank accounts, bribing police officers, procuring confidential data from the DVLA and phone companies, and trading sensitive material from live police inquiries.

Coulson has always insisted he knew nothing about the illegal activity which took place in the News of the World newsroom, telling MPs last year: "I have never had any involvement in it at all."

Mr A cannot be named now because he is facing trial for a violent crime, but his details will emerge once he has been dealt with by the courts. Coulson tonight refused to say whether he was aware of Mr A's criminal background, or of his return to the paper following his prison term. He said: "I have nothing to add to the evidence I gave to the select committee." ...