Image caption Glenn Mulcaire was jailed for six months in 2007 for phone hacking

Private investigator Glenn Mulcaire has lost his legal battle to conceal the identity of journalists who instructed him to hack into mobile phone messages.

Mulcaire, who worked for the News of the World, was jailed in January 2007 for unlawfully intercepting voicemail messages received by royal aides.

He appealed against giving names in proceedings brought by Nicola Phillips, assistant to publicist Max Clifford.

Five judges unanimously dismissed his appeal to the Supreme Court.

Mulcaire will now have to reveal the name to a "confidentiality club" made up of the lawyers and litigants in the civil phone hacking case.

He had said the appeal was made to protect his "legitimate legal interests" and that his lawyers had advised him he should not be forced by court orders to give "potentially incriminating answers" to questions asked in a civil case.

Ms Phillips's lawyer said the ruling was a "significant milestone" and would affect others who had sued News of the World publisher News Group Newspapers after claiming phones were hacked.

Mark Lewis said he expected Mr Mulcaire to serve a statement to his team within three weeks.

Notebooks surrendered

Mr Mulcaire's court battle began in May 2010 when Nicola Phillips, an assistant to publicist Max Clifford, launched a civil privacy case against News International - publisher of the now defunct News of the World.

Analysis Glen Mulcaire must reveal who commissioned him to hack Nicola Philips' phone. The Supreme Court says she was running a PR business, so the hacked messages amounted to commercially-sensitive information which amounts to intellectual property. But he won't have to reveal anything to claimants whose messages were purely private. The judgement appears to trump the normal legal rule that the law protects people against self-incrimination. So has the Supreme Court blown that legal protection away? No. The critical information on who commissioned Mr Mulcaire should remain confidential, revealed only to those involved in the hacking cases. The police may get to see it - but they won't be able to use the information to charge the private eye. However, detectives may be able to use it to better direct their investigations - and that, in time, could lead to charges.

She claimed voicemail messages left by her clients on her mobile phone had been unlawfully hacked and she applied to add Mr Mulcaire as a defendant.

She also applied for a court order that he should provide a witness statement revealing who instructed him to intercept messages.

Judges in the High Court and Appeal Court had earlier ruled Mr Mulcaire could be forced to answer questions about her claim.

Actor Steve Coogan is among others who are pursuing civil damages claims against News International.

Mr Mulcaire said in a statement he would "comply with the Supreme Court ruling to answer questions in Ms Phillips's case".

"I will consider with my lawyers what the wider implications of this judgement are if and when I am asked to answer questions in other cases," he added.

'Research assignments'

Mulcaire was jailed for six months in 2007, together with News of the World royal correspondent Clive Goodman, for his part in accessing messages left for members of the royal household.

The Supreme Court has ruled that Glenn Mulcaire cannot hide behind his right to silence Mark Lewis, lawyer for Nicola Phillips

At the time he surrendered notebooks to the police containing information about phone hacking.

In July 2011, it was alleged he also hacked into the mobile phone of murdered schoolgirl Milly Dowler.

Later that month, News International closed down the News of the World, after 168 years, as a result of the damning phone-hacking allegations.

In December, Mulcaire was rearrested on suspicion of conspiracy to hack into voicemail messages and perverting the course of justice. He was released on bail pending further inquiries.

Mulcaire was contracted by the News of the World to undertake "research assignments" from, or before, September 2001.