The Italian gossip magazine Chi has said it plans to publish photographs of the Duchess of Cambridge sunbathing topless in a special issue next week.
St James's Palace said on Friday it was suing the publishers of French magazine Closer which printed the pictures.
Closer and Chi are both part of former Italian Prime Minister Silvio Berlusconi's Mondadori media group.
No UK titles have published the photos, and the Daily Mail said papers in London had been offered similar images.
Chi's editor Alfonso Signorini said: "The fact that these are the future rulers of England makes the article more interesting and topical.
"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.
A royal spokesman described the photographs on Friday as a "grotesque invasion of privacy".
British newspapers were quick to condemn foreign titles that have published or plan to publish the pictures.
The Daily Mail, which revealed it had been offered pictures of the duchess, said that to have used them would have been a blatant breach of the editors' code of practice.
'Expectation of privacy'
The Times said that among the reasons no British newspaper had leaped to reproduce the pictures is that the British public would not forgive any newspaper that did so.
The Sun told its readers in an editorial that it would not be publishing the pictures, which it said "no decent British paper would touch with a bargepole".
It explained that, although it had recently published pictures of Prince Harry naked, this was a different situation, because the duchess had every expectation of privacy as she was on a private holiday in a private chateau as opposed to inviting strangers to a hotel suite.
"Nor is there the slightest public interest in publishing this set of intimate pictures in such circumstances," it added.
The Daily Mirror noted that the pictures were published by an officially regulated French press - whereas the self-regulated British press had not published them.
Closer, a celebrity gossip magazine, printed pictures of the duchess taken during the couple's private holiday in France.
A royal spokesman said the legal proceedings had been launched in France and were for breach of privacy.
Closer's editor said the couple were "visible from the street".
France has some of the world's strictest privacy laws but legal experts have suggested the magazine was likely to have weighed up the potential cost of a fine before publishing.
A spokesman for the duke and duchess strongly condemned publication of the pictures.
The couple were staying at the French chateau of the Queen's nephew, Lord Linley, when the photos were taken.
William and Catherine are currently on an official tour of South East Asia and the South Pacific to mark the Queen's Diamond Jubilee.
The couple spent Friday in the Malaysian capital of Kuala Lumpur where they started the day with a visit to a mosque for the first time.
They later left the mainland and flew to Kota Kinabalu, capital of the state of Sabah on Borneo, where they are visiting the Danum Valley Research Center to learn about rainforest wildlife.
Closer is not run by the same company as the British title.