The Middleton family has complained to the Press Complaints Commission after two newspapers published pictures of the Duchess of Cambridge and her sister Pippa in bikinis on a yacht.
The family decided to deliver a shot across the bows to tabloid newspapers, in a bid to prevent further intrusions of their privacy, following the wedding of Kate Middleton and Prince William last month.
The pictures, published in the Mail on Sunday and the News of the World, featured Kate, younger sister Pippa and their mother Carole, aboard a yacht moored off Ibiza.
The images, which show the family swimming, diving and sun-bathing, were taken by a photographic agency in 2006.
The News of the World story advertised topless pictures of Pippa Middleton, the subject of intense scrutiny since the wedding. The pictures and the story have now been removed from the paper's website.
The Middletons believe that the pictures are a breach of the PCC code, since a private yacht moored off an island could be a location where they had "a reasonable expectation of privacy".
The family is also concerned over a separate set of embarrassing pictures of Pippa and her brother James, circulating in the United States, which have been placed on the market by former friends. British newspapers, including the Daily Mail, have said that they will not publish the pictures.
The PCC previously brokered an agreement with newspaper editors not to "stalk" Kate Middleton, after she made a harassment complaint before her engagement to Prince William.
Newspaper editors have promised not to harass the Duke and Duchess on their impending honeymoon. The papers that printed the yacht shots could argue that the 2006 images had previously been seen and were already in the public domain.
No complaint is expected from the Middletons over pictures of the Duchess pushing an empty Waitrose trolley while shopping in Anglesey, which appeared in several tabloid newspapers last week.
Those pictures, accompanied by stories praising the "down to earth" Duchess, gave the couple's Clarence House handlers a PR coup and satisfied the media's demand for the first post-wedding pictures.
While the Duchess has a powerful media team around her, the Middletons are concerned about the sudden exposure of her younger siblings.
By taking up the Middleton complaint, the Press Complaints Commission hopes to demonstrate to public figures that it is an effective and cheaper alternative to applying for court injunctions.