Closer, a weekly round-up of celebrity gossip, ran a dozen shots of the duchess as she slipped off her bikini top, relaxed on a sun lounger and at one point pulled down the back of her bikini bottoms as William rubbed sun cream on her lower back.
Under the headline "Oh my God!", the photos show the couple, whose regal yet natural conduct since their April wedding has won them fans worldwide, soaking up the sun on the balcony of a 19th century hunting lodge, oblivious to lurking paparazzi.
The spread is a blow to Buckingham Palace as it tries to move on from a scandal over naked shots of Prince Harry that tarred an image bolstered by Kate and William's wedding, the Queen's 2012 Diamond Jubilee and her Olympic Games appearance.
While Closer defended its decision to publish the photographs, a royal spokesperson called it a "grotesque" move that had upset the royal couple, currently touring southeast Asia.
Britain's royal family says it is suing the French magazine that has published topless photos of Prince William's wife Kate.
St James's Palace said Friday that "legal proceedings for breach of privacy have been commenced today in France" by the royal couple.
"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," a spokesperson for St James' Palace said.
"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."
British Prime Minister David Cameron's spokeswoman also said the royal couple should be entitled to their privacy.
Closer's editor-in-chief Laurence Pieau described the photos as a "beautiful series" that showed a couple in love and were in no way degrading. She said the magazine had more intimate shots from the same series that it opted not to publish.
"There's been an over-reaction to these photos. What we see is a young couple, who just got married, who are very much in love, who are splendid," Pieau told French BFM television.
"She's a real 21st century princess," she added: "It's a young woman who is topless, the same as you can see on any beach in France or around the world."
WINDSORS TAKE THEIR CLOTHES OFF
Closer is published by Mondadori, an Italian company partly owned by former Italian Prime Minister Silvio Berlusconi and chaired by his daughter Marina. It would likely lose any legal case over invasion of privacy, although profits from the issue would likely far exceed any fine faced, probably just a few thousand euros.
"It's without a doubt an invasion of privacy," said Christopher Mesnooh, a U.S. lawyer who practices in France.
"They were on vacation in a friend's chateau. There was no expectation the press would be there. So visibly they have been damaged by the fact the pictures were taken and it is likely a French court would rule in that direction."
The publication -- which set off a stream of mostly angry commentary on twitter -- reopens a debate over the privacy of Britain's royal family and the freedom of the press weeks after a U.S. website published grainy photos of William's younger brother Harry cavorting naked in a Las Vegas hotel room.
"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 on the couple's mini-vacation in early September.
Many in Britain felt there was a difference, however, between exposing laddish antics by a royal in Las Vegas and intruding on William and Kate's private holiday.
"If she had taken her top off during a public engagement, it would be fair game, but they were on holiday, having personal time and so deserve privacy," said Shelley Brunton, 22, an office receptionist in London's financial district.
The editor of the Sun tabloid, the only British paper to run the nude Prince Harry photos, said that for the same reason it would not publish the shots of Kate. "The Sun has no intention of breaching the royal couple's privacy by publishing these intrusive pictures," editor Dominic Mohan said.
A torrent of twitter commentary on both sides of the Channel mostly chided Closer for running the shots.
"Utter disgrace that a mid-market French rag has invaded Kate Middleton's privacy like this.
Gutter press morality," tweeted billionaire media magnate Evgeny Lebedev, who owns British daily The Independent and London's Evening Standard.
"ANGER AND DISBELIEF"
The photos of Harry stained a positive image the Royal family has carefully crafted as it worked to turn the page on Princess Diana's death in 1997 and a raft of scandals at the time including Prince Andrew's ex-wife Sarah Ferguson having her toes sucked by a U.S. businessman as she sunbathed topless.
Royal sources said the photo spread felt like a flashback to the paparazzi chases of Princess Diana, William's mother, making it even more painful for him. "There is a feeling of anger and disbelief," one source said. "We feel there has been a red line crossed with regard to publishing these images."
"Have the paparazzi learnt nothing from Diana's death," Britain's MailOnline tweeted.
William and Kate were staying at the Chateau d'Autet near Aix-en-Provence in the Luberon region, whose picture-postcard villages, rolling lavender fields and vineyards have made it a favourite getaway spot for wealthy foreigners.
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 says: "People always say she doesn't need to dress up to look good. Well ... Kate is proving this."
Many in France -- where topless sunbathing is commonplace and Britain's royals are regarded with a mixture of fascination and bemusement given the French deposed their last monarch in 1870 -- could not see what all the fuss was all about.
"It doesn't shock me at all," said a newspaper vendor called Sylvie, selling the magazine in central Paris. "Like any woman in a private place, she should be able to do what she likes."
(With inputs from The Associated Press and Reuters)