A private investigator embroiled in a tabloid newspaper phone-hacking scandal today failed in a bid to keep secret the name of the journalist who instructed him to intercept messages.
The Supreme Court said Glenn Mulcaire, who worked for the now-defunct News of the World, must disclose the identity of the "person instructing him" to lawyers representing a woman who claims her phone was hacked and wants damages.
Mr Mulcaire argued that forcing him to reveal the name to Nicola Phillips, who was an assistant to public relations consultant Max Clifford, would be unfair because it might "expose him to prosecution".
But five Supreme Court justices - who said the case arose out of the phone-hacking scandal - today ruled against him following a hearing in London in May.
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.
"The Supreme Court has ruled that Glenn Mulcaire cannot hide behind his right to silence," added solicitor Mark Lewis after today's hearing.
"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.
"The Nicola Phillips judgment will have wide-ranging consequences in relation to all the phone-hacking claimants and beyond."
He said he expected Mr Mulcaire to pass information to Ms Phillips's legal team within the next three weeks.
Mr Mulcaire said in a statement issued by solicitors: "I will comply with the Supreme Court ruling to answer questions in Ms Phillips's case."
He added: "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."
Mr Mulcaire had appealed to the Supreme Court, the UK's highest court, after losing fights in the High Court and Court of Appeal.
He said his lawyers had advised him that he should not have to give "potentially incriminating answers" to questions asked in civil litigation, and added that he was fighting to protect his "legitimate legal interests".
Five years ago Mr Mulcaire and former News of the World royal editor Clive Goodman were jailed after the Old Bailey heard they plotted to hack into royal aides' telephone messages.
New police inquiries have started since then - separate to the launching of damages claims - and Mr Mulcaire has again been arrested, the Supreme Court heard.
A lawyer told judges Mr Mulcaire had been released on bail without charge pending further inquiries.