St James's Palace said Friday that "legal proceedings for breach of privacy have been commenced today in France" by the royal couple.
The French gossip magazine Closer published the blurry images Friday but insisted it had not invaded the couple's privacy.
The images are the first to show Britain's likely future queen partially nude.
The palace had earlier called the publication of the images a "grotesque" abuse of privacy. It likened the case to the hounding of William's mother, the late Princess Diana, by paparazzi before her untimely death in 1997.