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.

According to a spokesman, the pair were very upset that a French publication and a photographer have invaded their privacy in such a grotesque and totally unjustifiable manner.

Officials are consulting with lawyers on what options may be available to the couple.

William, who is second in line to the throne, and his wife 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 are not of the best quality but appear to have been taken with a long lens.

The photos cannot be accessed on the website of Closer in France, which shows only the magazine's front cover with heavily blurred-out images.

The photos come after just three weeks since 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.

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!"

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 that there had been no debate at the magazine over whether to publish the photos and that she believes they are not degrading, as "they are just like any other couple in love."

She described the reaction to the pictures' publication as "disproportionate" and slammed the British press as "complete hypocrites," since photos of Harry naked were published by The Sun.

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.