Private investigator Glenn Mulcaire has lost a legal fight to keep secret the identity of the person who instructed him to intercept mobile phone messages.
Mr Mulcaire, who intercepted messages while working for the News of the World, failed to persuade the Supreme 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 gave him the instruction to hack into phone messages.
Mr Mulcaire said his lawyers advised him that he should not have to give "potentially incriminating answers" to questions asked in court, and the appeal was made to protect his "legitimate legal interests".
Mr Mulcaire appealed to the Supreme Court after earlier losing in the High Court and Court of Appeal.
The five Supreme Court judges unanimously dismissed his appeal.
His battle began after Nicola Phillips, who was an assistant to public relations consultant Max Clifford, claimed her voicemail had been unlawfully intercepted.
In a statement issued following the court decision, Mr Mulcaire said: "As I made clear from the outset, I will comply with the Supreme Court's ruling to answer questions in Ms Phillip's case.
"I will consider with my lawyers what the wider implications of this judgement are if and when I am asked to answer such questions in other cases."
:: The court ruling came as police arrested a prison officer and two other people in connection with an investigation into corrupt payments made by journalists to officials.
Two men, aged 46 and 37, and a 50-year-old woman, were held at addresses in south-east London, Surrey and Kent.
The arrests are unrelated to the case against Mulcaire.
