The Metropolitan Police has won access to a witness statement filed by private investigator Glenn Mulcaire relating to who instructed him to intercept PR consultant Nicola Phillips's voicemail.
This follows a ruling in the Supreme Court earlier this month that Mulcaire cannot rely on privilege against self-incrimination.
Ordering disclosure of the statement to the Metropolitan Police, High Court judge Mr Justice Vos said disclosure "will not be unduly unfair to Mr Mulcaire or to any other potential defendants in the (phone hacking) case".
The court heard disclosure could open up new lines of inquiry in the high-profile phone-hacking litigation.
The Supreme Court ruled Mulcaire, who worked for the now-defunct News of the World, must disclose the identity of the "person instructing him" to lawyers representing Ms Phillips, an assistant to public relations consultant Max Clifford. She claims her phone was hacked and wants damages.
Mulcaire subsequently made a statement on 20 July, and today the Metropolitan Police sought access to it.
The application was opposed by Mulcaire, whose lawyers argued that being forced to disclose his statement to the police would be "unfair and oppressive".
But the judge said he had to take into account the public interest in investigating alleged serious crime.
He described the Mulcaire statement – its contents cannot be revealed, by order of the court – as "one brick in a very large wall".
The judge added: "Nevertheless the Mulcaire statement does contain positive information which may be of some benefit to the police investigation."
It would be "most undesirable" for the police service responsible for investigating criminal allegations of phone hacking to be "kept in the dark" regarding evidence that had come to light in civil proceedings.
The judge said investigations had already led to charges against eight people including former Sun editor Rebekah Brooks.
Jonathan Dixey, appearing for the Metropolitan Police, told him that officers were still considering bringing "further charges against further people – and doesn't rule out further charges against Mr Mulcaire".