The royal couple broke new ground by visiting a mosque for the first time during their tour of Malaysia, but all media attention was on the fact that another member of the royal family was at the centre of controversy over private images.
Closer magazine in France said the "exclusive" pictures showed Kate topless on the terrace of a guest house and were taken during a brief holiday she enjoyed with William in France last week.
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.
The Duke and Duchess of Cambridge tonight launched legal action against closer as St James's Palace compared the publication to the worst experiences of Diana, Princess of Wales at the hands of the paparazzi.
The palace led a chorus of protests, describing the invasion of privacy as "grotesque and totally unjustifiable", which had left Kate and William feeling "anger and disbelief".
In a short statement, the palace said of the planned action in the French courts: "St James's Palace confirms that legal proceedings for breach of privacy have been commenced today in France by the Duke and Duchess of Cambridge against the publishers of Closer Magazine France.
"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 royal couple have the sympathy of Downing Street with a source close to David Cameron saying that Number 10 "echoes the sadness of the palace" over the publication of the pictures.
But Laurence Pieau, the editor of Closer, was unrepentant, defending her decision to publish the pictures during an interview with the French radio station Europe 1, insisting there was "nothing degrading" about the photographs and claiming she could not understand the couple's reaction.
Ms Pieau also told the AFP news agency: "These photos are not in the least shocking. They show a young woman sunbathing topless, like the millions of women you see on beaches."
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.
William and Kate were told about the pictures today before they visited the Assyakirin Mosque and had also looked at the images on the website.
A source said: "We're not aware of anyone (in the UK) seeking to publish so the Press Complaints Commission is not coming into it."
The pictures are likely to reignite the controversy over privacy which raged around Prince Harry last month, when embarrassing images emerged of him frolicking naked in a Las Vegas hotel.
Staying in a £5,000-a-night hotel suite, Harry was filmed wearing a hat, sunglasses and colourful swimming shorts, and socialising with bikini-clad women at a pool party.
The Sun was the only British newspaper to defy a royal request issued via the Press Complaints Commission not to publish the photos of Harry in the nude with an unnamed woman.
Earlier there had been a security scare when a man believed to be an avid royal fan tried to approach the royals' chauffeur-driven limousine, apparently with the aim of handing the Duchess a flower or taking a photograph. He was detained by local police.
The emergence of the photos comes in the middle of William and Kate's so far successful Diamond Jubilee tour which has already taken them to Singapore and on to Malaysia yesterday.
At the mosque the couple arrived to find two seats waiting for them at the bottom of marble steps, so they could take off their shoes.
Kate wore a grey dress by Eulah London with a matching headscarf and in keeping with Islamic custom when visiting a mosque, her outfit had sleeves and the hem of the gown was below the knee.
The Duchess's LK Bennett heels slipped off easily to reveal stockinged feet, while the Duke had on black socks.
At the top of the steps they were greeted by the Imam of the mosque Ustaz Saiful, director of Islamic religious department Datuk Che Mat Bin Che Ali, chairman of the mosque Syed Abdullah, general manager of facilities Shausudin Ishak and head division development manager Datin Faudziah Ibrahim.
In front of them were row after row of pillars and the party strode through the one-and-a-half acre site built in 1997 which can cater for up to 12,000 worshippers at the height of Friday prayers, with space for a further 3,000 in the grounds.
Surrounded by swarms of press they walked up a flight of stairs to the main prayer space which was carpeted and brilliantly lit by a huge chandelier.
As they were guided into the large space, William asked "So is this where everyone gathers? When is your biggest service? How many people gather here?" When he was told as up to 12,000 on Fridays, he exclaimed: "Wow. So many."
The Duchess, whose scarf matched her outfit and framed her face, was deep in discussion with the development manager, telling her "It's so peaceful in here," adding: "It's really amazing."
When the manager mentioned she had been to Edinburgh University for her degree and masters, Kate said: "Oh wow, really? Well it's very cold there compared to here," and the two both covered their mouths giggling.
In front of them were five men sitting crossed-legged on the floor chanting passages from the Koran, which was held open on small tables in front of them.
The couple looked up at a dome above them which was decorated with writings from the Koran and through the windows of the roof could be seen the looming Petronas towers, Kuala Lumpur's famous landmark.
Before leaving the couple stopped to watch three men performing their ablutions - washing face, ears, hands and feet - which must be done before prayers can be made.
The trio were bent over running taps oblivious to the couple watching them complete the ritual.
The royal couple sat down at their starting point to replace their footwear and William handed Kate her shoes before tying his laces, and the Duchess joked with her partner telling him "mine are easy".
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 which makes the timing of the publication of the French pictures even more sensitive.
St James's Palace has confirmed that the topless pictures are of Kate but have yet to comment further about the images, which are likely to over shadow the royal couple's visit.
A source said the reaction by the Duke and Duchess to the images appearing was "anger and disbelief", although the couple, who have had a busy day of events in Kuala Lumpur, had not seen the actual topless photographs.
It added "They can't believe the pictures were taken, they can't believe anyone would publish them. The level of intrusiveness means a red line has been crossed."
The couple may seek redress thought the French legal system as the source added: "We are consulting with French lawyers at the moment."
The source said once they had a break in their schedule the couple would "turn their minds to what, if any, action they wish to take".
