The ramifications could also affect “kiss-and-tell” stories in print and on television, and could give stars the power to veto photographs taken in public. Publishers and media organisation are now mounting a legal battle against the “backdoor” assault on freedom of expression.

Loreena McKennitt, whose albums including The Book of Secrets and The Mask and Mirror have sold 13m copies worldwide, went to the High Court in London to stop a former friend from publishing a book about her. While the details of the case are not judged to be important, it was what Mr Justice Eady said in his judgment that has exercised legal minds.

They fear that the most trivial or anodyne details about a celebrity’s life, even ones that are known to the public, could now be hidden under the guise of protecting privacy.

Times Newspapers (publishers of The Sunday Times), other newspaper groups, the Press Association, the BBC, BSkyB and a number of magazine publishers will go to the Court of Appeal on September 4 to seek permission to intervene in the case.

They also fear public figures such as politicians and celebrities will use the case in an attempt to muzzle information that has already been made public. One media lawyer said: “It would be like trying to make someone a virgin again.”

Literary and other public figures who have fought to block unauthorised biographies include JK Rowling, Bono, Mary Archer and Sir John Mortimer.

Eady awarded McKennitt £5,000 damages and an injunction preventing Niema Ash from Hampstead, northwest London, publishing specific passages in her book Travels With Loreena McKennitt: My Life as a Friend. These included such mundane matters as what was under the lino of the house in Ireland, how many bunk beds were put up when visitors came to stay and what happened when McKennitt was aroused from sleep.

But his judgment went much further. For the first time a British court drew on a 2004 ruling at the European Court of Human Rights that said photographs of Princess Caroline of Monaco shopping in a public place or in a swimming costume at a beach club breached her right to privacy. The judge claimed there was a “significant shift” taking place between, on the one hand, the right of freedom of expression and the corresponding interest of the public to receive information and, on the other hand, “the legitimate expectation of citizens to have their private lives protected”.

He said information about an affair between two people could be protected even if one of them decided to reveal it to the public; incorrect information could breach someone’s right to privacy; and the fact that something was already in the public domain did not always mean it could be published again.

Ash has lodged an appeal and the media organisations are seeking to join in the action when it is heard later this year.McKennitt has said in an interview: “Privacy is integral to people’s emotional and psychological wellbeing. It doesn’t matter if you are a so-called public figure.”

Media lawyers say the case has wider ramifications than the long-running one brought against a tabloid newspaper by Naomi Campbell, the supermodel. She won £3,500 damages from the Daily Mirror after it revealed her fight against drug addiction. The Court of Appeal overturned the award but the House of Lords then allowed the model’s appeal against that judgment, saying the newspaper had gone too far in detailing her medical treatment.

Under the Eady judgment, celebrities will be able to sue for breach of privacy over the slightest affront to their feeling of self-importance. They will not have to prove that something is untrue, but just that raising it has invaded their privacy.

A spokesman for the solicitors Farrer & Co said: “This judge has clearly recognised the development of privacy cases in Europe. This judgment will be much quoted in future ‘kiss-and-tell’ actions.”

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