Police have raided the offices of the French magazine Closer, looking for details of the photographer who took the topless pictures of Duchess Kate, according to reports.
Parisian officers entered the headquarters at 10am on Wednesday morning, French newspaper Le Point reported.
The reports come a day after a French court ordered Closer to hand the snaps over within 24 hours, telling the publication that it would pay a €10,000 (£8,000) fine for every extra day they take to comply.
The Duke and Duchess of Cambridge have just completed a tour of South East Asia
Closer's parent company, Mondadori, owned by former Italian prime minister Silvio Berlusconi, will also have to pay €2,000 towards legal fees.
A court in Nanterre, near Paris, has now opened a separate criminal investigation into charges that Closer and a photographer breached the privacy of the duke and duchess by publishing the topless photos.
In the criminal suit, the magazine could be fined up to €45,000 (£36,000) and the editor, Laurence Pieau, could be jailed for up to a year.
Media law expert David Banks, a former co-author of editions of McNae's Essential Law for Journalists, told The Huffington Post UK that after Tuesday's ruling he wouldn't advise the pursuit of criminal charges against the photographers by Kate and William.
"I think it's a very dangerous path indeed if it comes out that a member of a European Royal Family is pursuing a reporter or journalist in order to imprison them. It will not play well with public opinion outside the UK."
French journalist Peter Allen has stressed the same point, tweeting:
He also pointed out that confidentially of sources, including photographers are protected under French Law.
However the snapper has been named by some British newspapers, which say the suspected photographer has worked for some of the biggest photo agencies in Europe.