The royal couple have already begun action against the French magazine Closer for publishing a dozen shots of Catherine, Duchess of Cambridge - the former Kate Middleton - taken as she slipped off her bikini top while sunbathing at a secluded French country house.
The Irish Daily Star published a two-page spread of 10 photographs of the duchess from Closer magazine under the headline "Angry Kate to sue mag over snaps". A teaser headline on the front page promised "the magazine shots everyone wants to see".
A spokeswoman for Prince William condemned the publications, saying: "There can be no motivation for this action other than greed."
An Italian gossip magazine said it would follow suit with a by publishing a 26-page reportage on the duchess on Monday that would include the pictures.
"I did this as a service to our readers, I'm a little taken aback by the reaction in the UK," Irish Daily Star editor Mike O'Kane told the BBC.
"It only seems to be an issue in the UK because she is your future queen. But from our point of view in Ireland, Kate Middleton is just another of the fantastic line of celebrities."
The British media group Northern and Shell, which owns 50 percent of the Irish Star, said it had no editorial control over the paper and accused it of a "grotesque" invasion of privacy.
The group, which publishes the Daily Express and Daily Star in Britain, said it was consulting lawyers over what it considered a "serious breach of contract".
The Irish media group Independent News and Media, which owns the other 50 percent, declined immediate comment.
The office of Prince William, second in line to the British throne, said in a statement that it would not comment on possible legal action other than saying "all proportionate responses" would be kept under review.
"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," it said.
The pictures have reignited a debate over privacy and freedom of the press, especially in Britain, where media could face new regulations after a series of publishing scandals.
All British papers have refrained from publishing the photographs, including the Sun, the only British title to run pictures of William's brother Harry naked in a Las Vegas hotel last month.
Italy's Chi magazine said its special edition on Monday would include some unpublished shots of the royal couple.
"The fact that we are dealing with the future British monarchs makes it certainly more interesting and in line with a modern conception of the monarchy," Chi Editor in Chief Alfonso Signorini said.
Both Chi and Closer are controlled by the Italian publisher Mondadori <MED.MI>, part of the media empire of former prime minister Silvio Berlusconi and chaired by his daughter Marina.
Closer's pictures, already circulating widely on the Internet, were also picked up by several foreign publications.
The Greek newspaper Eleftheros Typos had two photographs of the duchess, one showing her topless, on its front page.
"Her picture will be probably available online until the end of the earth. But it's not the end of the world," commentator Maria Laura Rodota wrote in Italy's biggest daily, Corriere della Sera.