The private investigator at the centre of the phone-hacking scandal has been ordered by a high court judge to reveal who instructed him to engage in the illegal interception of voicemail messages of public figures.

Glenn Mulcaire, who was jailed in January 2007 for intercepting the voicemail of eight people, had asked the court to rule that he should not have to answer questions because it might incriminate him.

But in the high court today Mr Justice Mann ruled that he must answer a list of questions about who instructed him to hack into the mobile phones of the celebrity publicist Max Clifford and his assistant, Nicola Phillips.

Mulcaire, who was employed by the News of the World at the time of his offences, is to be asked specifically whether he received instructions from the news editor of the paper, Ian Edmondson.

The judgment opens the door to the eventual disclosure of evidence that could have a powerful effect on News International, Scotland Yard, the Press Complaints Commission and the prime minister's media adviser, Andy Coulson, all of whom have claimed that Mulcaire acted without any official sanction from the News of the World.

The judge also ordered Scotland Yard to disclose paperwork that it has held since it raided Mulcaire's home and office in August 2005 which refers to Nicola Phillips or her mobile phone numbers.

At his trial in January 2007 Mulcaire admitted hacking into mobile phone messages left on Clifford's phone. When Clifford sued the News of the World last year for breach of privacy, Scotland Yard was ordered to disclose paperwork that was believed to identify a senior journalist who had ordered the hacking. However, Clifford withdrew his action in March before the disclosure was completed after the News of the World agreed to pay him more than £1m.

At the time of the hacking, in the spring of 2006, Phillips was working in Clifford's office. Following Clifford's legal action, she received information that she had also been a victim of Mulcaire's activity and started her own legal action against Mulcaire and the News of the World.

Lawyers acting for Mulcaire argued that he should not have to answer questions about the hacking of Phillips or any other alleged victim because this might tend to incriminate him. Mr Justice Mann agreed that Mulcaire might face further prosecution as a result of his answers but, on a technical point, he ruled that Mulcaire's evidence would be covered by section 72 of the Senior Courts Act, which requires a broad class of commercial evidence to be given regardless of any risk of self-incrimination.

The judge ordered that, within a timeframe to be agreed, the private investigator should disclose "the identity of the person or persons who instructed him to perform the exercise which resulted in him intercepting [Nicola Phillips's] mobile phone messages".

Among a series of questions that Mulcaire has been ordered to answer, the judge listed: "Did Ian Edmondson request him to investigate [Nicola Phillips]? Did Ian Edmondson request him to investigate individuals connected with Max Clifford?"

The judgment will affect a series of legal actions that are being brought by Mulcaire's alleged victims, including the former deputy prime minister John Prescott; the former deputy assistant commissioner of the Metropolitan police Brian Paddick; the actor Sienna Miller; and the Sky Sports presenter Andy Gray.

It was not known whether Mulcaire would seek to appeal against the judgment.

Separately, Mulcaire has been named as a witness due to give evidence in the criminal trial in Glasgow of the Scottish politician Tommy Sheridan, who is accused of perjury in relation to a libel action which he brought against the News of the World. Earlier this week Sheridan told the court that Mulcaire had been commissioned by the News of the World to intercept his voicemail.