The Duke and Duchess of Cambridge are furious after a French magazine published topless photos of Kate, calling it "grotesque and totally unjustifiable".
William is said to be particularly distraught because of the similarities between the violation of his wife's privacy and the media's treatment of his mother, the Princess of Wales.
"The incident is reminiscent of the worst excesses of the press and paparazzi during the life of Diana," a St James's Palace spokesman said in a statement.
"(It is) all the more upsetting to the Duke and Duchess for being so."
The future king was grim-faced as he walked the long red carpet to a plane at Kuala Lumpur airport on Friday afternoon, wrapping up a two-day stint for the couple in the capital.
They had been told the news over breakfast that the French magazine Closer planned to run the topless photographs, taken while they holidayed on a private estate in the country last month.
A royal source initially said the couple was "disappointed and saddened" and would consider legal action if the photos turned out to be genuine.
After the magazine hit newsstands later in the day he said they were "furious".
"Their sadness has turned to anger and disbelief as we learn more about the photographs," the source told media in Malaysia.
"We believe a real line has been crossed and we are consulting with French lawyers about what if any legal action will be taken."
Splashed on the front cover and inside on five pages, the shots show the duchess in just the bottoms of a black and white bikini.
One of the grainy shots shows William rubbing sunscreen on Kate's backside while in another she appears to be taking her bikini top off.
They ran with the line: "Victoria's Secret can stand aside".
The shots were taken while the couple were staying in the French chateau of the Queen's nephew, Lord Linley.
"Their royal highnesses had every expectation of privacy in the remote house," said the St James's Palace spokesman.
"It is unthinkable that anyone should take such photographs, let alone publish them ...
"(They are) hugely saddened to learn that a French publication and a photographer have invaded their privacy in such a grotesque and totally unjustifiable manner."
The spokesman said consultations were taking place with lawyers to work out what options were available.
The pictures are bound to restart the row over privacy that raged around Prince Harry last month when embarrassing images emerged of him frolicking naked at a Las Vegas hotel.
British newspapers were reportedly offered the shots of Kate last week but turned them down.
Despite the scandal erupting at home, William and Kate pressed ahead with plans to visit their first ever mosque on Friday.
In keeping with Islamic tradition, Catherine arrived at the site near Kuala Lumpur's landmark Petronas Twin Towers shrouded in a matching light-grey veil and floating chiffon dress by Beulah London.
After visiting the mosque, the couple were met by screaming crowds of more than 10,000 at Kuala Lumpur's KLCC park.
William and Kate shook hands and chatted with families who'd been waiting in the 30C degree heat since early morning.
The duchess's conservative outfit was remarkably similar to one worn by Diana on a visit to an Egyptian mosque during a tour of Egypt in 1992.
The Princess of Wales died in a Paris car crash in 1997 after being hounded for years by the media, even after her divorce from Charles and often while she was on holiday.
The photographs of Kate were taken in the same country, almost 15 years ago to the day after Diana's death.
"It's very disappointing; it's very saddening. We have turned the clock back 15 years," a royal source said on Friday.
"We have always maintained the position that the duke and duchess deserve their privacy, not the least when they are on holiday in their own swimming pool."
As the couple arrived at the airport on Friday, William was seen clenching his teeth and he didn't look to his left or right for most of the long walk to the plane.
Kate smiled only twice - once to him when they got out of their official car and then when she recognised newsmen on the tarmac.
As they went up the steps to board the the Boeing 737-800 jet, she reassuringly placed her left hand on the small of his back.