The private investigator jailed over phone hacking for the News of the World is taking legal action against the newspaper's publisher News International.
Glenn Mulcaire, who was jailed for six months in 2007 for intercepting messages on royal aides' phones, has lodged papers at the High Court.
A spokeswoman for News International, which owned the now-defunct Sunday tabloid, confirmed the action was being taken, adding that "the claim will be vigorously contested".
Neither News International or Mr Mulcaire's solicitor Sarah Webb, of Payne Hicks Beach, would give any further details about the nature of the action.
It came as the former US editor of the News of the World was quizzed by police over phone hacking.
James Desborough, who won a British Press Award in 2009, was arrested by appointment at a south London police station, sources said. He was quizzed for several hours about voicemail interceptions alleged to have taken place before his switch from London to Los Angeles, and later released on bail until October.
Desborough, who joined the News of the World (NOTW) as a showbusiness and news reporter in 2005 before being promoted to US editor in 2009, was arrested on suspicion of conspiring to intercept voicemails.
He is among journalists in the midst of a 90-day consultation period following News International's decision to kill off the paper in the wake of the phone hacking scandal.
Desborough, who was commended for a series of showbiz scoops including that TV host Fern Britton had a gastric band fitted, is the 13th person to be arrested as part of the fresh investigations into criminal activities at the Sunday tabloid.
A statement from News International said: "We are fully co-operating with the police investigation and we are unable to comment further on matters due to the ongoing police investigation."
