(CNN) -- A French magazine has stoked controversy by publishing pictures of Catherine, the Duchess of Cambridge, topless while on vacation with Prince William, prompting the threat of legal action from palace officials over what they blasted as a "grotesque" invasion of privacy.
The pictures were taken while the couple was staying at a private chateau in Provence, in southern France, according to Closer magazine.
A St. James's Palace spokesman said the pair were "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," the palace spokesman said.
Officials are consulting with lawyers on what options may be available to the couple, he said.
The pair remain focused on their tour on behalf of Queen Elizabeth II, he added.
William and Catherine are in Malaysia as they carry out a nine-day Southeast Asian tour of Commonwealth nations, which started in Singapore.
A royal source told CNN the Duchess of Cambridge is "upset" with the French magazine.
The photographs, which are grainy and appear to have been taken with a long lens, cannot be accessed on the website of Closer in France, which shows only the magazine's front cover with heavily blurred-out images.
St. James's Palace officials told CNN they "believe a red line has been crossed" and are consulting with French lawyers.
The photographs' publication comes only three weeks after the British royal family was caught up in a media furor over images of William's younger brother, Prince Harry, partying naked in his Las Vegas hotel room.
Those photographs were widely circulated online but were published in only one UK tabloid, The Sun. The press Complaints Commission, the UK press watchdog, received about 3,800 complaints from the public but said it was inappropriate for it to take any action in the absence of a formal complaint from the palace.
Laurence Pieau, editor-in-chief of Closer in France, posted several comments on Twitter late Thursday promoting the release of the topless pictures of Catherine.
One said: "The world exclusive of #closer tomorrow is Kate Middleton as you have never seen her ... and as you will never see her again."
Another read: "For kate middleton to have tan lines she would have to be wearing a swimsuit!"
Interviewed by CNN affiliate BFM-TV on Friday, Pieau defended the decision to publish the photographs, saying, "We were just doing our job."
The magazine was tipped off about the dates of the royal couple's visit to Provence and sent a photographer, she said. The pair were visible on a terrace from the road.
Pieau said she believes the photos are not degrading, as "they are just like any other couple in love."
She also slammed the British press as "complete hypocrites," since photos of Harry naked were published by The Sun.
She added that there are additional photos that are even more revealing. But, she said, Closer is not publishing them for now.
A string of negative comments from members of the public has already been posted on the French Closer magazine's Facebook page.
One, written in French, said: "Your behavior is unacceptable, you're as bad as the English tabloids. You have gone too far with the photos of Kate topless and it will cost you a lot I hope. Shameful."
The furor over the topless pictures of Catherine threaten to overshadow what has until now been a well-received tour in Asia, undertaken as part of the queen's diamond jubilee celebrations.
The Duchess of Cambridge gave her first public speech in Malaysia's capital, Kuala Lumpur, on Thursday, at a hospice for terminally ill children and adults. She is the patron of a charity supporting children's hospices in East Anglia, England.
The royal couple will be in Malaysia until Sunday before heading to the Solomon Islands and Tuvalu.
The Duchess of Cambridge was very conservatively dressed as she visited a mosque in Kuala Lumpur on Friday.
There was also a security scare when a man lunged at Catherine and tried to give her a flower as she and William left a festival of Malaysian dancing in a park in the capital.
A man was arrested over the incident, a palace source said.
"The Duchess was getting into the car and the Duke was walking round to the other side of the car when an enthusiastic member of the public tried to give her a flower," he told CNN.
"He was very close, right up against the car. He was arrested by local police officers. Royal protection officers were not involved, though they had noticed him in the crowd earlier on. The Duke and Duchess were not in any danger at all at any point."
William and Harry have been very sensitive to issues around invasion of privacy since the death of their mother, Diana, in a car crash as she fled a crowd of photographers in Paris 15 years ago.
It is not yet clear what form any legal action by the Duke and Duchess of Cambridge against Closer magazine, run by an Italian business called Mondadori, might take.
French magazine VSD was fined 2,000 euros ($2,580) last week after it published a photo of Valerie Trierweiler, partner of French President Francois Hollande, in a bikini on its front cover, BFM-TV reports.
Trierweiler is also taking legal action against other French magazines that published images of her and Hollande in swimwear while on vacation, BFM-TV says.
The French Closer magazine is run by a different company from the publication of the same name in the United Kingdom.
Closer UK, published by Bauer Consumer Media, issued a statement distancing itself from the French magazine.
"Closer magazine UK would like to make it clear that the two publications make entirely independent editorial decisions. In this respect the comments made by the editor of the French edition which have reported in the media today do not reflect the opinions of Closer magazine UK.
"Closer magazine UK was not offered any pictures of this nature and certainly has no intention of publishing the photographs of the Duchess of Cambridge which have been published in France this morning."
The British media is under close scrutiny after revelations of phone hacking and other abuses. The conclusions of an independent judge-led inquiry, which may recommend greater restrictions on media freedoms, are expected by the end of the year.