Popular gossip magazine Closer published the pictures on Friday, prompting a strong condemnation from the royal family.
The blurry photos show Kate wearing only a bikini bottom with her bosom exposed. The two were on a balcony at a private estate in Provence, a vacation spot near the French Riviera.
The royal family described the magazine's actions as a "grotesque and totally unjustifiable" invasion of privacy in the same country where William's mother Princess Diana died while fleeing paparazzi. The Duke and Duchess of Cambridge have filed a lawsuit against Closer.
In a statement released from a St. James's Palace official, the Duke and Duchess of Cambridge said they're 'hugely saddened' by the use of the photos.
"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."
Closer editor Laurence Pieau defended the decision to use the topless photos. She told French radio the couple was on a terrace that was visible from the road when they were photographed. She said they were not making an effort to conceal themselves and called the photos "joyful," not degrading. Pieau refused to say how much the magazine paid for the pictures.
William and Kate were touring the Far East and South Pacific to mark Queen Elizabeth II's Diamond Jubilee when the photos were published.
No major British publication carried the photos, including Rupert Murdoch's top-selling U.K. tabloid The Sun, which last month ran photos of a naked Prince Harry in a Las Vegas hotel room.
Anne Pigeon-Bormans, a French lawyer and an expert in media law, said the couple would have clear grounds for an invasion of privacy case. Last week, French first lady Valerie Trierweiler won a judgment of euro2,000 ($2,580) after the publication of photos of her in a bikini.
"French magistrates take into account the victim's behavior, when the person is flaunting themselves on camera. Kate Middleton will get damages because she's not behaving in this way," said Pigeon-Bormans.
It is common for women in Europe to sunbathe topless, both in private gardens and on public beaches.
The Associated Press contributed to this report.