Private investigator Glenn Mulcaire has lost a Supreme Court fight to keep the identity of the person who instructed him to intercept messages a secret.
Mulcaire, who intercepted mobile phone messages while working for the News of the World, failed to persuade the UK's highest court that he should be able to rely on privilege against self-incrimination in civil phone-hacking proceedings.
He had asked five Supreme Court justices to rule on whether he must disclose who instructed him.
Mulcaire said his lawyers advised him that he should not have to give "potentially incriminating answers" to questions asked in civil litigation at the High Court, and the appeal was made to protect his "legitimate legal interests".
Five years ago Mulcaire and former News of the World royal editor Clive Goodman were jailed after the Old Bailey heard that they plotted to hack into royal aides' telephone messages.
At the Supreme Court today, the five justices unanimously dismissed his appeal.
Mulcaire says he is at risk of being charged with criminal offences, and faces damages claims at the High Court, many of which have been brought by celebrities.
He fears he may incriminate himself if he answers questions in the civil proceedings.
Mulcaire appealed to the Supreme Court after losing fights in the High Court and Court of Appeal.
His battle began after Nicola Phillips, who was an assistant to public relations consultant Max Clifford, claimed her voicemail had been unlawfully intercepted.
Judges in the High Court and Appeal Court ruled that Mulcaire should disclose information relating to Phillips's claim.
Mulcaire was again arrested in December following the launch of a new police investigation, the Supreme Court was told at a hearing in London in May.
A lawyer told the judges Mulcaire had been released on bail without charge pending further inquiries.
Mulcaire said in a statement released before the start of the Supreme Court hearing: "The police have all the relevant documents from me since 2006 and I have already faced criminal proceedings, been convicted, and served a prison sentence.
"All the steps taken by my legal team in respect of the civil claims against me are to protect my legitimate legal interests."
Today's ruling was made by Lord Hope, Lord Walker, Lord Kerr, Lord Clarke and Lord Dyson.
Lord Walker, announcing the decision of the court, said the appeal "arises out of what has become known as the phone-hacking scandal".
Mulcaire 'will comply with the Supreme Court ruling'
After the hearing, in a statement issued through his lawyer, Mulcaire said: "I will comply with the Supreme Court ruling to answer questions in Ms Phillips' case.
"I will consider with my lawyers what the wider implications of this judgment are, if and when I am asked to answer questions in other cases."
In a statement issued through her lawyer, Phillips said: "This judgment will benefit a significant number of claimants who seek similar information."
Her solicitor, Mark Lewis, said: "The Supreme Court judgment is a significant milestone in the phone-hacking scandal.
"The Supreme Court has ruled that Glenn Mulcaire cannot hide behind his right to silence.
"He will now have to serve a witness statement answering questions about the News of the World journalist or journalists who gave him instructions and the nature of that information."
He added: "The Nicola Phillips judgment will have wide-ranging consequences in relation to all the phone hacking claimants and beyond."
Lewis said he expected Mulcaire would serve a statement to Phillips' legal team within the next three weeks.
Lord Walker said during 2005 and 2006 Mulcaire worked as a private investigator, "often engaged by staff on the News of the World Newspaper then published by News Group Newspapers Ltd (NGN)".
In January 2007, Mulcaire and Goodman pleaded guilty to offences relating to the interception of voicemail messages of the Royal Household and were sentenced to six and four months' imprisonment respectively.
Lord Walker said during 2008-2010 "a large number of civil claims were brought by individuals against NGN and some against Mr Mulcaire, claiming that messages on their mobile phones had been unlawfully intercepted".
In May 2010, Phillips began proceedings against NGN in relation to voicemail messages left on her mobile phone.
Part of her case was that the contents of voicemail messages left by clients on her mobile included "factual information, some of which is private information and some of which is commercially confidential information, including that relating to her clients' personal lives and relationships, health, finances, incidents in which the police have become involved, personal security or publicity issues, commercial business transactions, professional relationships and future career plans".
She applied to add Mulcaire as a defendant and for an order that he serve a witness statement disclosing information under several heads, including the identity of the person instructing him to intercept the messages.
He opposed the order for disclosure, relying on privilege against self-incrimination, on the basis that he could not be required to disclose that information as to do so would tend to expose him to prosecution.