Glenn Mulcaire, the private investigator at the centre of the News of the World phone-hacking case, has been ordered by the High Court to reveal the names of executives who commissioned him.

The court ruled that Mulcaire, whose contract with the Rupert Murdoch-owned tabloid was worth £100,000 a year, could not refuse to answer questions about his work on the grounds of self-incrimination.

In legal actions brought by the comedian Steve Coogan and the former Sky presenter Andy Gray, Mulcaire must now respond to inquiries about the names of News of the World journalists who ordered his services and the identity of celebrities whose phones were hacked.

Coogan is suing Mulcaire and the News International subsidiary News Group for breach of privacy for allegedly hacking into voicemail messages left on his mobile phone.

Mulcaire has already admitted passing phone intercept information to several individuals working on the News of the World news desk.

In another case, brought by the football agent Sky Andrew, Mulcaire was quoted in court documents as saying that he dealt with so many people on the news desk at the tabloid that he cannot recall precisely who received certain items of information.

Mulcaire was jailed in 2007, along with the News of the World royal editor Clive Goodman, for hacking into phones belonging to staff at Buckingham Palace. Mulcaire received a six-month sentence, while Goodman was sentenced to four months.

Lawyers for the Metropolitan police have claimed so many messages are being examined by the force's phone-hacking inquiry that it is difficult to identify every mention of a celebrity's name among "hundreds of intercepts".

The proliferation of legal actions generated by complaints against the News of the World is also in danger of congesting the courts with "parallel claims", the judge hearing applications for disclosure in three other cases has suggested.