Phone hacking was not confined to the News of the World but was widespread at other newspapers, including the Daily Mirror, a former Mirror reporter has claimed.

James Hipwell, who worked as a financial journalist under the editorship of Piers Morgan, said the practice was "seen as a bit of a wheeze" and offered to give evidence to the public inquiry into hacking ordered by David Cameron.

Hacking also took place at other titles in the newspaper group, including the People, he alleged.

"You know what people around you are doing," he told the Independent. "They would call a celebrity with one phone and when it was answered they would then hang up.

"By that stage, the other phone would be into [the celebrity's] voicemail and they would key in the code, 9999 or 0000. I saw that a lot."

He added: "It was seen as a bit of a wheeze – something that was slightly underhand but something many of them did. What a laugh.

"After they'd hacked into someone's mobile, they'd delete the message so another paper couldn't get the story. There was great hilarity about it."

Piers Morgan, a former editor of the Mirror, and the News of the World, has strenuously denied any link to the phone-hacking scandal.

Hipwell said he had chosen to speak out as he was sick of all the "lies".

The death of phone-hacking whistleblower Sean Hoare fuelled his decision to talk about what went on, he said, condemning the way the former News of the World journalist had been treated as "disgraceful".

Trinity Mirror denied its journalists had broken the law. A spokesman said: "Trinity Mirror's position is clear. Our journalists work within the criminal law and the PCC (Press Complaints Commission) code of conduct."

Trinity Mirror added that Hipwell's claims were "totally unsubstantiated".

Hipwell was jailed for six months in February 2006 for pocketing nearly £41,000 by repeatedly purchasing low-priced stocks, recommending them to readers in the Mirror's City Slickers column and then quickly selling them as values soared.