The publisher of Kate Middleton's infamous topless photos has been indicted. Publisher Ernesto Mauri has been formally charged with invasion of privacy.

Mauri is the Chief Executive of Mondadori France, the publisher of Closer, the French magazine that printed the topless photos in September.

The photographer who snapped the controversial picture, which showed the Duchess of Cambridge topless in a private resort in the South of France with husband Prince William last year, remains unnamed as the French media are protected from having to name their sources, including photographers.

However, another photographer, Valerie Suau of the regional newspaper La Provence, has also been charged with invasion of privacy for taking other photos of Kate in her swimsuit using a long lens camera. The photos she took of the Duchess did not contain nudity.

La Provence and Mondadori are similarly under police investigation.

"The photographer and legal representatives of both companies have been questioned concerning the photographing of a person on private property and the criminal use of these images," a spokesperson for the prosecutor in Nanterra court said.

"The maximum penalty for this kind of offence is one year in prison and a fine of 45,000 euros [approximately AUD56,800] for individuals. For corporate bodies, the fine is 45,000 euros, cessation of business for five years, and public notification of the decision."

The royal couple file legal action against Closer for printing their private photos, describing the act as a "grotesque breach of privacy."

St. James Palace also immediately released a statement after the publication of the said pictures, saying, "Their Royal Highnesses have been hugely saddened to learn that a French publication and a photographer have invaded their privacy in such a grotesque and totally unjustifiable manner. The incident is reminiscent of the worst excesses of the press and paparazzi during the life of Diana, Princess of Wales, and all the more upsetting to the duke and duchess for being so."

The French court is expected to reach a decision on June 5.