A furious Duke and Duchess of Cambridge launched legal action against a French magazine tonight after it published topless pictures of Kate taken while the couple were on holiday.
The publication was compared by St James's Palace to the worst experiences of Diana, Princess of Wales at the hands of the paparazzi.
The palace led a chorus of protests, describing the invasion of privacy as "grotesque and totally unjustifiable".
Royal aides drew parallels between Diana's most upsetting encounters with certain elements of the press and the "unthinkable" actions of the French magazine Closer, which left Kate and William feeling "anger and disbelief".
And tonight the palace announced that lawyers would be pursuing the matter through the French courts.
In a short statement, the palace said: "St James's Palace confirms that legal proceedings for breach of privacy have been commenced today in France by the Duke and Duchess of Cambridge against the publishers of Closer Magazine France."
The royal couple have the sympathy of Downing Street with a source close to David Cameron saying that Number 10 "echoes the sadness of the palace" over the publication of the pictures.
In a strongly-worded statement St James's Palace said: "Their Royal Highnesses have been hugely saddened to learn that a French publication and a photographer have invaded their privacy in such a grotesque and totally unjustifiable manner.
"The incident is reminiscent of the worst excesses of the press and paparazzi during the life of Diana, Princess of Wales, and all the more upsetting to the Duke and Duchess for being so.
"Their Royal Highnesses had every expectation of privacy in the remote house. It is unthinkable that anyone should take such photographs, let alone publish them.
"Officials acting on behalf of their Royal Highnesses are consulting with lawyers to consider what options may be available to the Duke and Duchess."
But Laurence Pieau, the editor of Closer, was unrepentant, defending her decision to publish the pictures during an interview with the French radio station Europe 1, insisting there was "nothing degrading" about the photographs and claiming she could not understand the couple's reaction.
Ms Pieau also told the AFP news agency: "These photos are not in the least shocking. They show a young woman sunbathing topless, like the millions of women you see on beaches."
William and Kate are midway through a Diamond Jubilee tour of the Far East, which had been going well until now.
The photographs were taken last week while the couple were staying in Provence at a chateau owned by Lord Linley, the Queen's nephew, ahead of their trip.
St James's Palace said the royal couple would not let the controversy distract them.
A spokesman said: "The Duke and Duchess remain focused currently on their tour of Singapore, Malaysia, Solomon Islands and Tuvalu on behalf of HM the Queen."
A source added: "Their sadness has turned to anger and disbelief as we learn more about the photographs."
The royal couple had spent the day in the Malaysian capital of Kuala Lumpur completing a busy schedule of events which saw them break new ground by visiting a mosque for the first time.
They later left the mainland and flew to Kota Kinabalu, capital of the state of Sabah on Borneo, and tomorrow will travel to the region's dramatic rainforest to learn about the wildlife - something that is likely to be a welcome relief from the events of today.
The publication of the pictures was described as "nauseating" by Michael Ellis, the Conservative MP for Northampton North, who said the incident would bring back painful memories for William and his brother Prince Harry, as their mother Diana had died following a car crash in France when the vehicle was being pursued by paparazzi.
Mr Ellis said: "It is no more than voyeurism. It is in my view frankly close to criminal conduct. If a person was in a state of undress and had photographs taken of them, they would be rightly offended and appalled, and quite frankly the person taking the photograph could be subject to arrest. I don't see the difference in this case."
Publishers of the UK edition of Closer distanced themselves from the French magazine.
Chief executive Paul Keenan of Bauer Media said his company deplored the publication of the "intrusive and offensive pictures" and had "complained in the strongest terms" to the firm which licensed the title in France.
He said Bauer had asked Closer France to remove the pictures and refrain from publishing any more.
Legal experts said the royal couple would have a strong case.
Thomas Roussineau, who specialises in privacy law, said publication of the photos undoubtedly breaks French privacy laws .
"It is totally forbidden," he said. "The castle is not the street, it is in a private place, and they are intimate pictures."
But he said it was likely the magazine had weighed up the potential cost of a fine against the revenue the photos would bring.
Caroline Jan, solicitor at London-based firm Kingsley Napley's media group and active member of the Franco-British Law Society, said it would be the "biggest Franco-British privacy clash since Princess Diana's death".
She added: "The French magazine publishing pictures of the Duchess is clearly testing the water in a country where privacy laws are stricter than in the UK."
But media lawyer Mark Stephens suggested William and Kate might not have the ability to take effective action over the photographs.
He said: "It is obviously highly intrusive but as they have published the pictures the genie is out of the bottle."
A source said the legal proceedings were the result of William and Kate feeling they had to make a stand following the actions of the magazine, but the controversy would not stop them getting the most out of their tour on behalf of the Queen.
The source said: "This is a clear and unjustifiable, grotesque breach of privacy. If we don't take a stand against this, then when would we make a stand?"
He added: "They feel very strongly about what's happened in France today, but it's no word of a lie they are upbeat and are enjoying the tour.
"They remain utterly committed to their work and this will not intrude on it." end