Palace officials say the Duke and Duchess of Cambridge have been left saddened their privacy has been breached by photographs that allegedly show Kate topless.
French magazine Closer has published the pictures today.
A St James's Palace spokesperson said the couple are "very angered" and feel a "red line has been crossed".
The palace has said it is consulting French lawyers.
Officials said the matter "turns the clock back 15 years" - when Diana, Princess of Wales, was still the subject of intense media scrutiny.
The magazine says the images show the Duchess topless on the terrace of a guest house.
The magazine's French website showed an image of its new front cover with a heavily pixellated photograph of the Duchess in a bikini apparently about to remove her top.
But the pictures were not pixellated in the magazine when it was published this morning.
And the publication's editor, Laurence Pieau, wrote on her Twitter page: "We can say that after tomorrow's Closer, Harry will feel less alone ..."
The pictures appear to have been shot using a long-lens camera from a distance and were allegedly taken while Kate and William were on holiday in France last week.
William and Kate were told about the pictures today before they visited the Assyakirin Mosque in Kuala Lumpur and had also looked at the images on the website.
Palace officials told Sky News: "The Duke and Duchess are saddened their privacy has been breached. It's turned the clock back 15 years.
"They woke up to the story this morning (and) have seen the photos, which were taken at a private hotel by a private swimming pool."
The Duke and Duchess were staying in Provence at a chateau owned by Lord Linley, the Queen's nephew, ahead of their current Diamond Jubilee tour of south east Asia and the South Pacific on behalf of the Queen.
During the visit to the mosque earlier Kate wore a head scarf as she and her husband were greeted by the Imam.
Following Islamic rules, the couple removed their footwear before being shown inside.
They listened to readings from the Koran and were old about practices of Islamic observance, including preparations for prayer and the role of the mosque in the community.
Kate appeared unfazed as news of the pictures spread around the world. If she had been embarrassed then it didn't show.
And later a relaxed Duchess smiled and waved at Malaysians who had gathered to see her at a park in central Kuala Lumpur.
She stopped, chatted and joked with her husband as they made their way on foot towards a conference centre where the Duke delivered his first speech in the country.
Publication of the images revives the privacy row prompted by photographs that emerged last month of Prince Harry frolicking naked at a Las Vegas hotel.
The Sun was the only British newspaper to defy a Press Complaints Commission advisory note not to publish photos of the prince naked with an unnamed woman.
The Duke and Duchess arrived in Singapore on Tuesday and are currently in Malaysia before travelling on to Borneo, then the South Pacific.
Malaysia is a largely Muslim country with laws on public decency, making the timing of the pictures' emergence even more difficult.
----
Typical french.