JOIN USFormer police officer Derek Webb secretly videoed Mark Lewis, the lawyer who represented the family of Milly Dowler, a murdered teenage girl whose phone was hacked by the now-defunct paper, the Guardian newspaper and the BBC reported.
The surveillance was an attempt by the tabloid to show Lewis was having a relationship with lawyer Charlotte Harris, who also represented hacking victims, and was sharing confidential information, said the BBC.
Webb also reportedly spied on Harris, Lewis's former assistant.
The surveillance was carried out last year.
The allegation is the latest twist in the hacking scandal which forced the closure of the Rupert Murdoch-owned News of the World in July and has sent shock waves through his News Corporation media empire.
It heaps pressure on the Australian-born media mogul's son and heir apparent James Murdoch as he prepares to be questioned for a second time by a committee of British lawmakers on Thursday about the affair.
Webb, who ran a private investigation firm called Silent Shadow, told the BBC he was commissioned by the tabloid to carry out surveillance on Lewis and Harris in early 2010.
He said he was paid to go to Manchester, where the lawyers were based, and once followed Lewis's former wife and his daughter, filming them when they visited shops and a garden centre.
According to the Guardian, the News of the World also took advice to see if it could injunct Lewis and prevent him from representing alleged hacking victims and tried to persuade one of his former clients to sue him.
News International, Murdoch's British newspaper wing which published the News of the World until its closure, did not immediately have any comment.
Lewis has been the most prominent lawyer representing alleged victims of hacking.