Prince William and his wife Catherine Friday reacted with fury after a French magazine published topless photos of her, saying it raised the spectre of press harassment of William's mother Diana.
The young British royals, who are currently in Malaysia, were consulting lawyers over what they called a "grotesque" breach of privacy by the French version of Closer magazine, palace officials said.
The world exclusive grainy pictures show Catherine (30) wearing just the bottoms of a black and white bikini and having suncream rubbed onto her behind by William while on holiday in the south of France.
"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 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."
The statement said officials acting on behalf of the couple "are consulting with lawyers to consider what options may be available to The Duke and Duchess."
"The Duke and Duchess remain focused currently on their Tour of Singapore, Malaysia, Solomon Islands and Tuvalu" to mark Queen Elizabeth II's diamond jubilee, it said.
Royal aides told AFP that William, the second in line to the British throne, and Catherine felt "anger and disbelief" and felt that a "red line has been crossed" by the publication of the pictures.
The incident "turns the clock back 15 years," one source said, referring to the intense media attention on Diana Princess of Wales, who died in a car crash while fleeing paparazzi in Paris in 1997.
The pictures are a further headache for the royal family, already reeling from the recent publication of nude photos of William's younger brother Harry in a Las Vegas hotel.
Closer magazine invited readers on its website to pick up he new edition to enjoy "the photos that the world can't wait to see; the Duchess of Cambridge topless on a guesthouse terrace".
Many of the grainy photos feature Catherine wearing just the bottoms of a black and white bikini and looking so toned that the magazine said "Victoria's Secret angels can stand aside."
In one, William is rubbing sunscreen on the lower end of Kate's backside, in another the couple laugh as they lounge in chairs near a folded red parasol, in a third Catherine holds the top of her bikini across her chest, either taking it off or putting it back on.
Closer magazine said the snaps were taken on the terrace of the Autet Chateau in the southern French region of Provence where William and Catherine were said to have vacationed last week.
The magazine said the chateau was owned by Viscount Linley, the son of the queen's late sister Princess Margaret.
The three-day holiday began at Marseille-Marignane airport on 5 September and the mood was so relaxed that "Kate even smoked a cigarette between the airport's exit and the car that was to take them toward 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, it 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."
British newspapers reportedly turned down offers to buy the pictures.
The photo row threatened to overshadow the couple's nine-day visit to Asia, which was meant to help introduce the pair overseas after their pomp-filled marriage last year.
It is part of a tour by Britain's younger as part of celebrations marking the 60-year reign of William's grandmother the queen.