Pieau added that she found the pictures of Kate far tamer than those of a naked Prince Harry in Las Vegas hotel suite that were published in Britain's The Sun tabloid last month.
A French lawyer who is an expert in media law said the royal couple had clear grounds for an invasion of privacy case against the magazine.
"French magistrates take into account the victim's behavior, when the person is flaunting themselves on camera. Kate Middleton will get damages because she's not behaving in this way," said the lawyer, Anne Pigeon-Bormans.
If the couple's lawsuit reaches court and if the magazine is found guilty, it could face a fine of up to €45,000 (about $60,000). Potential criminal sanctions include up to a year in prison, according to French law. Last week, French first lady Valerie Trierweiler won a judgment of €2,000 ($2,580) after the publication of photos of her in a bikini.
The British media, wary about an ongoing U.K. inquiry into suspected criminal wrongdoing at a number of papers, has generally respected the palace guidelines.
"There's absolutely zero chance of the British press publishing these photos," Neil Wallis, the former deputy editor of the Murdoch's News of the World, told AP.
Wallis, who was arrested last year in the British phone hacking scandal, said the arguments against publication under British rules are many: Kate had an expectation of privacy, she was doing nothing wrong and she was photographed by stealth by someone using a long lens.
In contrast, he said, publishing the naked photos of Prince Harry was legitimate because they raised questions about his judgment and the security arrangements around the third in line to the British throne.
Associated Press writers Raissa Ioussouf in London, and Lori Hinnant, Thomas Adamson and Angela Charlton in Paris contributed to this report.
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