The House of Lords has turned down a petition for permission to appeal in the privacy battle between Canadian folk singer Loreena McKennitt (pictured) and Niema Ash, the former friend who wrote a book about her.
Mark Thomson, a partner with solicitors Carter-Ruck, who represented McKennitt, said: "It is probably the most important decision for 20 years.
This is a landmark decision for protection of privacy in this country.
"With an intense focus on the facts of the case at trial, Mr Justice Eady performed a proportionate balancing exercise on the parties' respective rights.
"The judgment has helped define the breadth of privacy rights, the relevance of accuracy in the material disclosed and the extent to which the public interest is truly served by mere curiosity."
McKennitt said: "I am very grateful to the courts, including the House of Lords, the Court of Appeal and Mr Justice Eady who have recognised that every person has an equal right to a private life."
Ash said she was "devastated" by the House of Lords' decision, which she felt had ignored major legal issues. She said she would now seek to take the case to the European Court of Human Rights.
Hugh Tomlinson QC, of Matrix chambers, a leading expert on privacy law, said: "It is very significant that the House of Lords has refused permission to appeal. This means that radical new developments in privacy law established by that case are now firmly established."
David Price Solicitors and Advocates, which represented Ash in the Court of Appeal, argued that the decisions by the lower courts represented "a significant shift in favour of privacy at the inevitable expense of freedom of expression".
Media groups see the decision as opening the gates for the courts to issue more pre-publication injunctions, with wider effects.
The case also suggests that publication of private information about public figures will be allowed only if it makes a contribution to a debate of public importance, irrespective of whether the information is already in the public domain.
Media law observers say they fear that many personalities will now try to argue that news organisations and others intend to publish "false private information" about them, thus giving the court a route by which to issue an injunction, rather than to claim that they intended to publish defamatory material, which would not.
Many celebrities go to court, they say, not to protect their reputations, but in order to safeguard their public images from publication of material which might show them in a bad light.