The publisher of a French celebrity magazine was placed under formal investigation for breach of privacy after the publication of topless pictures of Britain’s Duchess of Cambridge, a judicial source said.

The photographer who snapped the pictures of Kate Middleton last summer and a regional daily were also put under investigation, the last step in France before being charged, the source said.

Closer magazine, a weekly round-up of celebrity gossip, published in September a series of photos of the Duchess, the wife of Prince William, topless on holidays in southern France, something that incensed much of the British public.

The photos showed Kate slipping off her bikini top, relaxing on a sun lounger and at one point pulling down the back of her bikini bottoms. Buckingham Palace had called the photo spread a “grotesque” invasion of the royal couple’s privacy.

The publisher of the magazine – the French branch of Italy’s Mondadori – and the paparazzo who snapped the photos from a distance were put under formal investigation earlier this month, the judicial source said, speaking on condition of anonymity.

Mondadori in a statement said the company “was not aware of any new developments in addition to what is already known on this matter”.

The regional daily La Provence, which published pictures of Prince William and his wife in swimming suits during the same holiday, was put under formal investigation this week, the source said.

An investigation does not necessarily lead to formal charges.

The office of Prince William and the Duchess of Cambridge could not be reached for comment.