The publication was compared by St James's Palace to the worst experiences of Diana, Princess of Wales, at the hands of the paparazzi.
The palace led a chorus of protests, describing the invasion of privacy as "grotesque and totally unjustifiable".
Royal aides drew parallels between Diana's most upsetting encounters with the press and the "unthinkable" actions of the French magazine Closer, which left Kate and William feeling "anger and disbelief".
The palace announced that lawyers would be pursuing the matter through the French courts.
In a short statement, the palace said: "St James's Palace confirms that legal proceedings for breach of privacy have been commenced today in France by the Duke and Duchess of Cambridge against the pub- lishers of Closer Magazine France."
The royal couple have the sympathy of Downing Street with a source close to David Cameron saying No 10 "echoes the sadness of the palace" over the publication of the pictures.
St James's Palace said: "Their Royal Highnesses have been hugely saddened to learn that a French publication and a photo-grapher 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 unthink-able that anyone should take such photographs, let alone publish them."
However, Laurence Pieau, editor of Closer, was unrepentant, defending her decision to publish the pictures during an interview with the French radio station Europe 1, insisting there was "nothing degrading" about the photos.
She added: "These photos are not in the least shocking. They show a young woman sun- bathing topless, like the millions of women you see on beaches."
William and Kate are midway through a Diamond Jubilee tour of the Far East.
The photographs were taken last week while the couple were staying in Provence at a chateau owned by Lord Linley, the Queen's nephew, ahead of their trip.
St James's Palace said the royal couple would not let the controversy distract them.
A spokesman said: "The Duke and Duchess remain focused currently on their tour of Singapore, Malaysia, Solomon Islands and Tuvalu on behalf of HM the Queen."
The royal couple had spent the day in the Malaysian capital of Kuala Lumpur completing a busy schedule of events, during which they visited a mosque for the first time.
Publishers of the UK edition of Closer distanced themselves from the French magazine.
Chief executive Paul Keenan of Bauer Media said his company deplored the publication of the "intrusive and offensive pictures" and had "complained in the strongest terms" to the firm that licensed the title in France.
He said Bauer had asked Closer France to remove the pictures and refrain from publishing any more as legal experts said the royal couple would have a strong case.
Thomas Roussineau, who specialises in privacy law, said publication of the photos undoubtedly breaks French privacy laws.
"It is totally forbidden," he said. "The castle is not the street, it is in a private place, and they are intimate pictures."
He added it was likely the magazine had weighed the potential cost of a fine against revenue generated by the photos.
Caroline Jan, solicitor at London-based firm Kingsley Napley and an active member of the Franco-British Law Society, said it would be the "biggest Franco-British privacy clash since Princess Diana's death".
She added: "Publishing [these] pictures of the Duchess is clearly testing the water in a country where privacy laws are stricter than in the UK."
However, media lawyer Mark Stephens suggested William and Kate might not have the ability to take effective action over the photographs.
He said: "It is obviously highly intrusive but as they have published the pictures the genie is out of the bottle."
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