Prince William and his wife Catherine are to sue after a French magazine printed topless photos of her, saying it evoked painful memories of press harassment of William's mother Diana.
"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 couple's office said in a statement.
The young British royals, who are in Malaysia to mark Queen Elizabeth's Diamond Jubilee, attacked what they called a "grotesque" breach of privacy by the French version of Closer magazine, palace officials said.
But the editor of the magazine said the photos were "not in the least shocking".
The grainy pictures show Catherine, 30, wearing just the bottoms of a black and white bikini and having suncream rubbed on to her behind by William while on holiday in the south of France.
Earlier, the couple's office 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 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."
The publication claimed they were taken on a guesthouse terrace in France where the couple were on holiday this month.
But no British papers have so far picked up the shots.
"In this country no editor, I promise you, is even beginning to think about publishing these pictures," Neil Wallis, the former executive editor of Rupert Murdoch's now defunct News of the World tabloid, told Sky News.
The couple were particularly upset because the pictures wound back the clock 15 years to the media hounding of Princess Diana before her death, the Press Association said. She died in a car crash in Paris in 1997 while photographers chased her.
Bloomberg, Associated Press, Agence France-Presse, Reuters