The British royal family on Saturday warned an Italian magazine that "unjustifiable upset" would be heaped on Prince William's wife Catherine if it went ahead and printed topless photos of her.
The royals launched legal action on Friday against French magazine Closer -- which is part of the media empire of former Italian prime minister Silvio Berlusconi -- after it became the first publication to print paparazzi snaps of Catherine sunbathing with her husband.
The royal family hoped to contain the scandal by swiftly suing Closer -- but Chi, an Italian weekly also published by Berlusconi's Mondadori Group, announced late on Friday that it planned to print the same images on Monday.
"Any such publication would serve no purpose other than to cause further, entirely unjustifiable upset to the Duke and Duchess of Cambridge, who were enjoying time alone together in the privacy of a relative's home," said a spokeswoman from the couple's office at St James's Palace.
The palace would not comment on potential legal action concerning the planned publication in Italy, but warned that "all proportionate responses will be kept under review".
Closer magazine confirmed to AFP that a hearing would be held on Monday at a court in Nanterre, north west Paris, on the complaint.
William, second-in-line to the throne, and Catherine on Saturday put on a brave face for the cameras during a visit to a Borneo jungle.
A version of the front cover of Chi showed some of the photos, taken while the couple were on holiday in France this month, with the headline "The Queen is Naked!"
"The fact that these are the future rulers of England makes the article more interesting and topical," editor Alfonso Signorini told reporters.
"This is a deserving topic because it shows in a completely natural way the daily life of a very famous, young and modern couple in love," he said.
William and Catherine, both 30, are suing Closer for invasion of privacy over the pictures which show Catherine sunbathing in just a bikini bottom by a pool.
The images evoked memories of the press harassment of William's mother Diana, who died in a 1997 car crash as she was pursued through Paris by paparazzi.
William is believed to blame the paparazzi for his mother's death.
Palace officials said the royals considered the pictures published by Closer to be a "grotesque" breach of privacy.
"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," St James's Palace 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."
Pieau has insisted Closer has no regrets about printing the pictures.
"These photos are not in the least shocking. They show a young woman sunbathing topless, like the millions of women you see on beaches," she told AFP.
Closer said the pictures were taken on the terrace of the Autet Chateau in Provence, southern France. The magazine said the chateau is owned by Viscount Linley, the son of Queen Elizabeth II's late sister Princess Margaret.
The palace said the royal couple "remain focused" on their tour of Singapore, Malaysia, the Solomon Islands and the tiny Pacific island of Tuvalu to mark the diamond jubilee of William's grandmother Queen Elizabeth II.