- Catherine is "upset" by the claims in the French magazine, a royal source says
- Palace: "We believe a red line has been crossed. We are communicating with lawyers"
- The topless photographs are grainy and appear to have been taken with a long lens
- The royal couple is currently on a tour of southeast Asian nations
(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.
St. James's Palace told CNN the photos appeared to be genuine. "We believe a red line has been crossed. We are communicating with French lawyers," officials said.
A royal source told CNN the Duchess of Cambridge is "upset" by the claims in 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.
A St. James's Palace spokesman said: "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."
The pair remain focused on on their tour on behalf of Queen Elizabeth II, the statement said.
Laurence Pieau, editor-in-chief of Closer, posted several comments on Twitter late Thursday promoting the images' release.
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!"
The photographs' publication comes only three weeks after the British royal family was caught up in a media furore over images of William's younger brother, Prince Harry, partying naked in his Las Vegas hotel room.
William and Catherine are currently in Malaysia as they undertake a nine-day southeast Asian tour of Commonwealth nations, which started in Singapore.
The Duchess of Cambridge gave her first public speech in Kuala Lumpur 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 the capital, 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.
A palace source told CNN: "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 tried to run forward and throw it into the car. 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."
Closer in France is not affiliated to the magazine of the same name in the United Kingdom.
CNN's Saskya Vandoorne contributed to this report