Officials said that a criminal complaint was lodged by Prince William and his wife Catherine with French prosecutors over topless photos of Catherine published in a gossip magazine.
The legal action by the royal couple's lawyers came as an Italian magazine on Monday splashed the pictures across a special edition with a front-page headline that read: "The queen is nude!"
The prosecutor's office in the Paris suburb Nanterre said it received the complaint against persons unknown with which the pair aim to get compensation for the images of Catherine sunbathing semi-naked at a French chateau.
The royal couple have said they want damages for the alleged breach of French privacy law from both Closer magazine and from the photographer, whose identity the publication has not revealed.
The couple are also seeking an injunction to stop Closer magazine re-selling the images of Catherine in bikini bottoms. The injunction request was to be considered at an emergency hearing in a Nanterre court from 1600 GMT on Monday.
The prince is furious over the images, which drew comparisons with press harassment of his mother Diana who died in a Paris car crash in 1997 while pursued by paparazzi.
No newspaper or magazine in Britain -- whose racy, mass-selling tabloids have frequently been accused of unwarranted intrusion into the lives of the rich and famous -- has announced plans to publish the offending photographs.
But the possibility of legal action failed to intimidate Irish or Italian titles, with the pics of Catherine sunning herself in bikini briefs appearing Saturday in a Dublin tabloid and Chi magazine publishing them on Monday.
Chi, part of the Mondadori group, which is owned by Italy's former prime minister Silvio Berlusconi, produced a special edition featuring the grainy photos along with a series of articles on topics such as "Kate's breasts, natural or fake?"
The Mondadori group is chaired by Berlusconi's daughter Marina Berlusconi, who on Sunday defended running the pictures as a matter of editorial freedom.
The magazine's editor Alfonso Signorini argued in the special edition that the pictures represented "extraordinary reportage."
"For the first time, the future queen of England was appearing in a natural way, without the constraints of etiquette," he wrote.
In 2006, Chi sparked outrage in Britain when it printed a photo of a fatally injured Princess Diana being given oxygen at the scene of the high-speed crash in a Paris road tunnel in 1997, together with details from her autopsy.
A royal spokeswoman declined to comment Sunday on whether the royal family was launching legal action against either Chi or the Irish Daily Star.
"All proportionate responses will be kept under review," she said.
Unlike Mondadori, the two media groups that jointly own the Irish Daily Star condemned its decision to run the pictures.
Britain's Northern and Shell group said it was taking "immediate steps" to shut down the joint venture with Dublin-based publisher Independent News and Media which runs the Irish Daily Star.
Last month photos emerged of William's brother Prince Harry cavorting naked with women at a Las Vegas party.
The two images of Harry are widely available on the Internet but in Britain only The Sun newspaper published them.
The palace has said the couple, who were in the Solomon Islands on Monday, "remain focused" on their nine-day southeast Asian and Pacific tour marking Queen Elizabeth's 60 years on the throne.