The Duchess of Cambridge is said to be "saddened" after a French magazine published topless photographs of her, Royal officials said.
The magazine, Closer, said the pictures were taken when Prince William and Catherine were on a romantic break last week.
In a fresh blow to the Royal family, which is still reeling from the scandal over naked shots of Prince Harry partying in Las Vegas, the magazine published a series of images of the Duchess sunbathing topless.
Closer, a weekly round-up of celebrity gossip, ran a five-page spread of photos of Catherine relaxing topless with Prince William on a balcony at a 19th century hunting lodge in southern France, said to be owned by a son of the late Princess Margaret.
Under the headline 'Oh my God!', the photos show the Duchess removing her bikini top then relaxing on a sun lounger, apparently oblivious to paparazzi lurking nearby as the pair vacationed at the property in early September.
"Harry started the fashion: these days the Windsors take their clothes off," the French magazine quipped in the photo spread, which it claimed as a world exclusive.
At one point the Duchess, clearly recognisable in the slightly fuzzy images, stands up and partly peels down her bikini bottoms as William rubs sun lotion on her back.
"The last time we saw Kate and William on a balcony it was for their wedding. But they had more clothes on," reads one photo caption.
Another said: "People always say she doesn't need to dress up to look good. Well ... Kate is proving this."
Closer also said Kate was spotted smoking a cigarette as the couple walked out of nearby Marseille airport where they arrived on a commercial flight
The couple were staying in Provence at a chateau owned by the Queen's nephew, Lord Linley, ahead of their tour of south-east Asia and the South Pacific, when the shots were taken.
It is an area famed for picture-postcard villages, rolling lavender fields and vineyards which have made it a favourite getaway spot for wealthy foreigners.
Royal officials said they were not in a position to verify if the photographs were genuine, at this stage, reported the BBC.
Although sources told Reuters they believed they were genuine.
The magazine pre-announced its intention to publish the photographs, with a teaser image on its website which showed the Duchess apparently poised to take off her bikini top, boasting it had images of her "topless on the terrace of a guest house in Luberon".
In a blurb below the images, it said: "A little more than a year after their marriage, the royal couple was offered a romantic getaway, far from the protocol and etiquette in their very own garden of Eden.
"Almost alone in the world ... because Closer was there!" it said.
"Discover the incredible pictures of the future Queen of England as you've never seen & and as you never will see her again!".
William and Catherine were informed of the magazine's plan to publish the images, reportedly while they were having breakfast in Kuala Lumpur, Malaysia.
Officials said the couple were "saddened and disappointed" by the news, which they consider to be a breach of privacy.
A source said Buckingham Palace planned to contact French lawyers to see what options were available.
"There is a feeling of anger and disbelief about these photographs," the source said.
"We feel there has been a red line crossed with regard to publishing these images."
There was no immediate comment from Closer magazine.
BBC Royal Correspondent, Peter Hunt, said if the pictures are genuine, their publication would be "a bitter blow" to the couple.
"Prince William witnessed the complex relationship his mother, Diana, had with the media," he said.
"He believes the actions of the paparazzi in Paris contributed to her death.
"He wants to shield his wife from the worst excesses of reporters and photographers in full flight."
The images have emerged only weeks after Prince Harry was snapped naked while in Las Vegas.
Speaking ahead of the publication, Laurence Pieau, editor of French Closer magazine, said Prince Harry would feel "less alone" when the magazine hits shelves on Friday, and the pictures are beamed around the world, the Daily Mail reported.
She said the images showed Catherine "fully topless" and that there would be "nipples".
The publication is sure to reopen the debate over the privacy of Britain's Royal family and the freedom of the press.
The pictures of the Duchess were clearly taken on private property using cameras with extremely long lenses, which meant no British newspaper would publish them, according to the Daily Mail.
In the meantime, the couple have continued with their tour, which is part of the Queen's Diamond Jubilee celebrations.
In Kuala Lumpur, Catherine gave her first public speech while abroad, at a hospice.