The Duke and Duchess of Cambridge have said they are "hugely saddened" by what they said was a "grotesque" invasion of their privacy after topless photographs of Kate were published by a French celebrity magazine.
In a statement issued on Friday morning, St James's Palace said the move by Closer magazine was "reminiscent of the worst excesses of the press during the life of [Princess] Diana" and lawyers were being consulted.
The incident may yet have wider political implications, as Closer magazine in France is owned by Mondadori, a publishing company owned by the former Italian president Silvio Berlusconi
The royal couple, who are touring south-east Asia, are "furious" and "very very angry" according to close aides travelling with them in Malaysia on Friday, after the magazine published what it described as exclusive pictures of Kate topless under the headline: "Seulement dans Closer: Kate et William, leurs vacances tres hot en Provence."
The officials said a red line had been crossed in terms of the couple's privacy .
The pictures were apparently taken on the terrace of a guest house during a brief holiday with William in France last week.
Royal photographer Harry Page told Sky News that they were taken on a 640 acre estate on the south of France and from what he had seen they were taken with long lenses and the couple would have had no idea they were being taken.
In an extraordinary move, which underlined the fury of the royal couple, St James Palace issued a strongly-worded statement.
"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," it said.
"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 Duke and Duchess remain focused currently on their tour of Singapore, Malaysia, Solomon Islands and Tuvalu on behalf of the Queen."
The couple were staying in Provence at a chateau owned by Lord Linley, the Queen's nephew, ahead of their diamond jubilee tour of south-east Asia and the South Pacific on behalf of the Queen.
They did not know anything about the photos until the magazine's website showed an image of its new front cover with a heavily-pixellated image of a woman with dark hair that it claimed was the Duchess, in a bikini, apparently about to remove her top.
On Friday morning, the magazine emerged on the streets of Paris with the pictures of Kate.
Sources said that royal officials did not make contact with Closer in Paris last night, because the magazine had already come off the printing presses.
But St James's Palace was consulting French lawyers, with royal officials saying the publication was "turning the clock back 15 years" to when Princess Diana was pursued by paparazzi.
It was reported that the photos were offered to British papers last week, but they all turned them down.
However, one picture desk executive on a national tabloid said the set of photos being touted around last week were different. "They were also long lens, but you couldn't see anything. These pictures nobody has seen, as far I am aware," the executive said.
Page, a photographer who has worked with national newspapers for the past 30 years, said: "From what I have seen, these photos have been taken from a very long way. Kate and William would have had no idea they were being taken.
"They were on a 640 acre estate in the south of France. I think they would have expected a certain degree of privacy. They were on a private holiday.
"Remember the toe-sucking photos of Fergie, again in the south of France. That is exactly 20 years ago this month and there was a scramble for them. But now there is not a single newspaper in Britain who would publish these pictures."
The publication of the topless pictures of Kate will not just reignite memories of Princess Diana but will also feed into the Leveson report on the ethics of the press.
Lord Justice Leveson is currently drafting his final report and recommendations and this controversy will underline the difficulties any future British regulator will have in controlling overseas press and internet publication.
Closer's audacious decision to go to press comes just weeks after the US showbusiness website TMZ published photos of a naked Prince Harry cavorting in a Las Vegas hotel room.
William and Kate were told about the allegations on Friday morning before they visited the Assyakirin Mosque and had also looked at the images on the Closer website.
Speaking about the royal couple, a source said: "They're saddened their privacy has been breached – if it has been breached.
"We will talk to our lawyers in London and counterparts in Paris to see what options are available. We're not aware of anyone [in the UK] seeking to publish so the Press Complaints Commission is not coming into it."
The Sun was the only British newspaper to defy the palace's request not to publish photos of Harry in the nude with an unnamed woman.
Prince Charles had issued the request to newspapers, via the PCC, along with a letter from his personal lawyers at Harbottle & Lewis warning them that he had an expectation of privacy under English law, saying there is no possible justification for printing the photos apart from prurience.
In 1994 the German Bild magazine published photos of Prince Charles naked on a balcony while on holiday at a French chateau near Avignon, but no British papers published them.
French Closer was launched in 2005 as a version of the British magazine then owned by UK publishing company Emap. It was sold in 2006 to Mondadori France, a subsidiary of an Italian company, and sells 400,000 copies a week.