A lawyer representing phone hacking victims says he has seen documents suggesting that News International placed politicians under surveillance.
Mark Lewis, who was himself tailed by a private detective hired by the media company, said Labour MP and hacking campaigner Tom Watson was among those named in a report he was given by police.
Mr Lewis told Victoria Derbyshire on BBC Radio 5 Live that he planned to bring a civil claim for breach of privacy against News International, former publishers of the News of the World.
The solicitor said documents suggested that the move to place politicians under surveillance was ordered "by someone at News International".
"I have seen Tom Watson's name, and other politicians' names have featured. This is a report that I have been handed by the police," he said.
Mr Lewis's clients include the family of murdered schoolgirl Milly Dowler, whose voicemails were illegally intercepted by the News of the World after she went missing in 2002.
News International said yesterday that it was "deeply inappropriate" that it hired private investigator Derek Webb to spy on Mr Lewis and fellow lawyer Charlotte Harris, who also represents victims of phone hacking.
The company declined to comment further on Mr Lewis's latest claims.
Mr Lewis condemned the spying, which included following and filming of his former wife and children.
He said: "It's disgraceful. No father wants his child surveyed - this is wrong on so many levels. It thoroughly undermines any notion of fair play.
"I am sure they were doing it in order to stop me and Charlotte Harris representing our clients. It was undoubtedly a fishing exercise."
Mr Webb, a former police officer who ran a private investigations firm called Silent Shadow, told the BBC he had not been paid compensation he was owed for his loyalty to the News of the World over eight years of service.
The private detective said he was commissioned by the now-closed Sunday tabloid to carry out surveillance on Mr Lewis and his former assistant Ms Harris in Manchester early last year, while James Murdoch was executive chairman of News International.
Mr Watson sits on the Commons Culture, Media and Sport Committee, which is investigating phone hacking and will hear further evidence from Mr Murdoch on Thursday.