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.
Mr 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.
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 Mr Mulcaire should disclose information relating to Ms Phillips's claim.
In a statement issued through her lawyer, Ms 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.
In a statement issued through his lawyer, Mr Mulcaire said: "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."
Five years ago Mr 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.