A French judge will lead a probe into how topless photos were obtained of Prince William's wife Catherine and will seek to identify the photographer, sources close to the inquiry said Friday.
French prosecutors last week ordered a preliminary investigation into whether Closer magazine, which published the images on September 14, and the photographer had committed a criminal act.
That has now become a fully-fledged investigation, the sources said.
The management of the French magazine was unreachable for comment on Friday evening.
An injunction granted by a court in Nanterre in the Paris suburbs had ordered the magazine to hand over all forms of the pictures to the British royal couple within 24 hours or face a 10,000-euro ($13,000) fine for every day's delay.
It also bans the glossy magazine from re-using them in print or on its website and re-selling them on pain of further 10,000-euro fines for each infraction.
After their debut in the French weekly, the photos of the Duchess of Cambridge sunbathing topless have appeared in magazines in Denmark and Sweden, Ireland's Daily Star and Italy's Chi, which like Closer is owned by former prime minister Silvio Berlusconi's Mondadori media group.
The pictures were taken when the royals were vacationing in southern France at a chateau owned by Viscount Linley, the son of Princess Margaret, the deceased sister of Britain's Queen Elizabeth II.
Closer magazine has said its editor Laurence Pieau received death threats for publishing the photographs. After publication, Pieau had defended them saying they were not in the "least shocking".
"They show a young woman sunbathing topless, like the millions of women you see on beaches," she told AFP.
The royal family's lawyers have also sought criminal charges against the unidentified photographer, in a bid to curb the spread of the pictures.
Closer has said it does not own the images and simply bought them for exclusive first use, so it likely does not possess all the original files. It has refused to say from whom it bought them or who the photographer is.
But French celebrity photographer Pascal Rostain, who has had several scoops, told AFP that Closer had commissioned the snaps, which were taken by an Irish photographer.
About the photographer's identity, he said: "In our small paparazzi world, we know who it is but obviously don't say anything.
"I can just say it is an Irish national who lives in the south of France," he said.
Rostain also said Closer would have to shell out far more money if it had to buy the pictures from a photographer or agency which had rights to them.