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.