The magazine, Closer, said it had exclusive pictures of Kate topless on the terrace of a private chateau owned by the Queen's nephew Lord Linley.
Its website also showed an image of the front cover with the Duchess in a bikini apparently about to remove her top.
The pictures of the Duchess of Cambridge were taken with a long lens, at a private home
Speaking about the royal couple a source said: "They're saddened their privacy has been breached - if it has been breached."
The source stressed it could not confirm if the pictures were of Kate as they appeared to have been taken with a long lens and were pixellated.
The source added: "We will talk to our lawyers in London and counterparts in Paris to see what options are available."
He went on to say: "We're not aware of anyone (in the UK) seeking to publish so the Press Complaints Commission is not coming into it."
The BBC's royal reporter Nicholas Witchell said it appeared British newspapers were offered the alleged photographs of the couple last week, but turned them down.
Closer magazine published a preview of the pixellated photos on its website
The pictures were allegedly taken while Kate was on holiday with her husband, the Duke of Cambridge, in France last week.
The couple were staying at the Provence chateau, ahead of their current Diamond Jubilee tour of south-east Asia and the South Pacific on behalf of the Queen.
The publication would restart the row over privacy which raged around Prince Harry last month, when embarrassing images emerged of him frolicking naked in a Las Vegas hotel.
The Sun was the only British newspaper to defy a Press Complaints Commission advisory note not to publish photos of Harry in the nude with an unnamed woman.
The Mail said that the pictures of the Duchess were clearly taken on private property using cameras with extremely long lenses, which means no British newspaper would publish them.
It said the timing of publication could not be worse, with the Cambridges almost midway though a hugely successful tour on behalf of the Queen to celebrate her Diamond Jubilee.
They arrived in Singapore on Tuesday and are currently in Malaysia before travelling on to Borneo then the South Pacific.
The Duchess of Cambridge visited the Assyakirin Mosque in Kuala Lumpur, wearing a headscarf and removing her shoes
Malaysia is a largely Muslim country with laws on public decency which makes the timing of the photos even more difficult.
At the Assyakirin Mosque in Kuala Lumpur on Friday the couple arrived to find two seats waiting for them at the bottom of marble steps, leading up to the domed place of worship, 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 nude 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.
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 with the royal telling her "It's so peaceful in here", adding: "It's really amazing."
When she 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 help open on small tables in front of them.
The royals 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 royals stopped to watch three men performing their ablutions - washing face, ears, hands and feet - that must be performed before prayers can be made.
The trio were bent over running taps oblivious to the royals 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".
Later on Friday, the royal couple will experience the full diversity of Malaysian life when an outdoor extravaganza is staged in their honour.
They will enjoy a cultural fair of music and dance, in the capital Kuala Lumpur, reflecting all aspects of the Commonwealth country's heritage.
Later, at a lunch given by the British Malaysian Chamber of Commerce, trade links between the two nations will be highlighted by the Duke when he gives a speech to the assembled businessmen and women.
He is expected to underline the importance of the relationship both for British and Malaysian prosperity and for cultural ties between the two nations.
During the day, William and Kate will also enjoy a Diamond Jubilee tea party hosted by Simon Featherstone, the British High Commissioner to Malaysia, and his wife Gail at their residence.
At the end of the day, the royal couple will leave mainland Malaysia and fly to Kota Kinabalu, capital of the state of Sabah on Borneo, from which they will travel into the rainforest jungle tomorrow to explore the habitat.
