The palace has already launched a civil lawsuit against France's Closer magazine, which published the paparazzi snaps of Kate, the Duchess of Cambridge, relaxing during a holiday at a private villa in Provence.
The couple is hitting back hard against publication of the pictures, which William's office called a "grotesque" abuse of the young royals' privacy.
A spokeswoman for William's St. James's Palace office said the couple's lawyers would file a complaint with French prosecutors on Monday against the unidentified photographer or photographers involved.
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