An angry Prince William and his wife Catherine launched legal action on Friday against a French magazine that printed topless photographs of her.
The publication of the pictures of the former Kate Middleton, taken while she and the second in line to the throne were on holiday in the south of France, evoked memories of the press harassment of William's mother Diana.
Palace officials said the royals, who are currently visiting Malaysia, considered the pictures published by the French version of Closer magazine to be a "grotesque" breach of privacy.
But the editor of the magazine defended the decision to publish the photographs, telling AFP they 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 backside by William at the side of a swimming pool.
"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," their office at St James's Palace said.
"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."
Later, the royal family confirmed 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 palace said the royal couple "remain focused" on their tour of Singapore, Malaysia, the Solomon Islands and the tiny Pacific island of Tuvalu to mark the diamond jubilee of William's grandmother Queen Elizabeth II.
But royal aides told AFP that William and Catherine felt "anger and disbelief" and believed that a "red line has been crossed".
The incident "turns the clock back 15 years," one source said, referring to the intense media attention surrounding Diana, Princess of Wales, who died in a car crash while fleeing paparazzi in Paris in 1997.
William is believed to blame the paparazzi for his mother's death.
British Prime Minister David Cameron's spokesman said the royal couple were "entitled to their privacy." A source close to Cameron said he "echoes the sadness of the palace".
The pictures are a further headache for the royal family, already reeling from the publication last month of nude photos of William's younger brother Harry in a Las Vegas hotel.
Laurence Pieau, the editor of Closer, insisted she had no regrets about the pictures.
"These photos are not in the least shocking. They show a young woman sunbathing topless, like the millions of women you see on beaches," she said.
In one of the shots, the couple are laughing as they lounge in chairs near a folded red parasol. In another, Catherine holds the top of her bikini across her chest, either taking it off or putting it back on.
Closer said the snaps were taken on the terrace of the Autet Chateau in Provence, where William and Catherine were said to have spent time last week before leaving for Asia.
The magazine said the chateau is owned by Viscount Linley, the son of the queen's late sister Princess Margaret.
The three-day holiday began at Marseille-Marignane airport on September 5 and "Kate even smoked a cigarette between the airport's exit and the car that was to take them towards their little paradise," the magazine wrote.
Comparing the pictures to those of Harry cavorting naked with a mystery woman as he played strip billiards in Las Vegas, the magazine said: "Here there is no question of alcohol, of strip billiards or compromising positions, but simply a spouse at ease with her body, who has nothing to hide from her husband.
"Moments of grace, of complicity. Of love in its purest. Images of cloudless happiness."
Rupert Murdoch's top-selling British tabloid The Sun, the only British paper to publish the pictures of Harry, said it would not print the photographs of Catherine, and there was no sign of any other British publication doing so.
"The Sun has no intention of breaching the royal couple's privacy. The circumstances are very different to those relating to the photos of Prince Harry in Las Vegas," editor Dominic Mohan said.