The News of the World hired a specialist private investigator to run covert surveillance on two of the lawyers representing phone-hacking victims as part of an operation to put pressure on them to stop their work.

The investigator secretly videoed Mark Lewis and Charlotte Harris as well as family members and associates. Evidence suggests it was part of an attempt to gather evidence for false smears about their private lives.

The News of the World also took specialist advice in an attempt to injunct Lewis to prevent him representing the victims of hacking and tried to persuade one of his former clients to sue him.

The surveillance of Lewis and Harris occurred during the past 18 months, when Rupert Murdoch's son James was executive chairman of the paper's parent company, News International. He is due to give a second round of evidence to a House of Commons select committee on Thursday, and is likely to face intense questioning about the quality of his leadership.

Neither lawyer would comment but friends say they are furious at what they see as an attempt at "blackmail" and are considering suing the News of the World for breach of privacy. They have previously had to reassure clients that their private lives would not be exposed if they dared to sue the paper.

A News International spokesperson said : "News International's enquiries have led the company to believe that Mark Lewis and Charlotte Harris were subject to surveillance. While surveillance is not illegal, it was clearly deeply inappropriate in these circumstances. This action was not condoned by any current executive at the company."

Lewis and Harris have been part of a small group of lawyers who have mounted a series of devastating legal actions against News International. Separately, they represented Gordon Taylor and Max Clifford, the first two victims to sue the company for hacking their phones.

Harris also acts for football agent Sky Andrew, whose case led in January to the resignation of the prime minister's media adviser, Andy Coulson. Lewis also represents the family of the murdered schoolgirl Milly Dowler, whose case led to the closure of the News of the World in July.

Emerging evidence suggests the lawyers were targeted on at least two occasions by Derek Webb, an investigator who specialises in physically following people and in making secret videos of their movements. Webb has worked for the News of the World since 2003, following hundreds of targets including members of the royal family and serving cabinet ministers. Emails recovered by Scotland Yard disclose the names of those working for News International who hatched the plans.

Webb was given the job as part of an attempt to prove a false claim that Harris was having an affair with a Manchester solicitor and other false claims about the private life of Harris and her children. It is not yet clear exactly how the News of the World would have used the information if any claim had proved to be true.

In spring 2010, following a hostile report by the Commons media select committee, the News of the World hired Webb to gather evidence on Lewis. For reasons that are not yet clear, he focused on Lewis's former wife and secretly filmed her home in Manchester, following her and taking further video of her and her daughter as they visited local shops and a garden centre.

She told BBC2's Newsnight it was "absolutely terrifying to think that somebody can be watching you, following you around" and that she now felt nervous and scared when she left her house.

In January 2011, Webb was hired to spy on Harris. This was at a time when the case of her client, Andrew, had uncovered information that led to the sacking of the paper's news editor, Ian Edmondson.

Webb was asked to find evidence that she was having an affair with a Manchester solicitor. The allegation was false; Harris had never met the solicitor in question.

Other investigators were hired to supply reports on the two lawyers, although it is not clear who commissioned them. One of the reports clearly suggests that somebody had been following Harris and her two young children.

In evidence to the media select committee in September, the News of the World's in-house lawyer, Tom Crone, was asked by the Labour MP Tom Watson if he had seen dossiers on the private lives of claimant lawyers.

Crone said: "I saw one thing in relation to two of the lawyers, except I do not know whether it was a dossier. It involves their private lives."

He suggested he could not name those who had commissioned this work without interfering with current police inquiries.

Separately, according to internal emails recovered by Scotland Yard, the News of the World commissioned a senior barrister to advise on whether they could injunct Lewis to stop him working for any alleged victim of phone hacking on the grounds that he had confidential information from his work for Gordon Taylor.

The newspaper's solicitors, Farrer and Co, wrote to Lewis threatening to injunct him if he took on any hacking clients but took no action when Lewis ignored the threat.

The internal emails also reveal that the newspaper's lawyers tried to approach solicitors acting for Lewis's former client Gordon Taylor to see if they could persuade him to sue Lewis. This also failed, and Lewis has gone on to represent several dozen clients who are suing the News of the World for their alleged role in hacking their phones.

Webb, a former police officer, is now in dispute with the paper and has sought the help of the National Union of Journalists to pursue a claim that it failed to honour an agreement to give him a loyalty payment after the paper closed in July.

Webb is known to have followed members of the royal family, often on instructions from the former royal correspondent Clive Goodman, who was jailed in January 2007 for intercepting the voicemail of three members of the royal household. Webb also followed John Prescott when he was deputy prime ministerand Charles Clarke, the former home secretary. The newspaper continued to hire him after the phone-hacking scandal broke, and he is known to have been following a leading trade unionist shortly before the paper closed.

In November 2008, Webb was cleared of aiding and abetting misconduct in public office in a controversial case in which Thames Valley police arrested a local newspaper journalist, Sally Murrer, and tried to have her prosecuted for receiving information from a police officer.

Physical surveillance is not normally seen as a criminal offence but it is possible that Webb's targets might sue for breach of privacy.

Tom Watson, who sits on the Commons' culture select committee investigating phone hacking, told Newsnight that the News of the World hired the private investigator in a bid to "besmirch" the victims' lawyers.

He said: "I think it shows an utterly relentless and ruthless organisation, clearly highly politicised and who would stop at nothing to try and cover this case up.

"It's yet another revelation that I think will shock people when they get to know what it means."

James Murdoch, the executive chairman of News International, is due before the committee again on Thursday.

"He's got some very serious questions to answer," said Mr Watson, "because his original testimony has not just been contradicted by the former editor of the News of the World but it's been contradicted by Tom Crone, the in-house lawyer, and I am sure we will want to go into great detail about how his recollection of events differs to a number of other people who work for him."