The Duke and Duchess of Cambridge are to sue over images of Kate Middleton topless published in French Closer.
Clarence House announced the decision following talks between lawyers and royal aides.
Closer published shots of the royal couple taken with a long lens camera. She is topless and rubbing sun cream into her husband's back in the images, by the pool at a remote house deep in the countryside in southern France.
A Royal aide called the snaps a "grotesque and unjustifiable invasion of privacy."
The editor of French Closer Ms Laurence Pieau defended them as "not degrading at all." She claimed the pair were "visible from the street."
Why have the Royals chosen to take legal action now, but did nothing when Harry was pictured naked in a Las Vegas?
Significantly, Harry was perceived as being partly cupable for allowing the images to be taken during a party in a plush hotel.
In contrast, the Duke and Duchess of Cambridge picked a very secluded spot in France.
Deep in the Provence countryside at a chateaux owned by William's cousin, it is very far removed from the bright lights of Vegas.
Also, France has very tough privacy laws governing what can and cannot be published.
Lawyers for the young couple must believe they have strong grounds for success.
So if they chose to not sue over the publication of the images, the question would arise of precisely how far a publication could go in publishing snaps of private moments.
So this legal action could be intended for the media as a warning by William and Kate - who have discussed this week wanting two children together - to the press that they mean business and will not let their lives be reduced to a freak-show.