People.com said the unauthorized, long-lens shots were taken on the private island of Mustique last week.
"We are disappointed that photographs of the duke and duchess on a private holiday look likely to be published overseas," a spokesman for the couple at St. James's Palace told the celebrity magazine Tuesday. "This is a clear breach of the couple's right to privacy."
Photos of the duchess sunbathing topless in France while on vacation with her husband, Prince William, were published about six months ago.
The couple married in April 2011 and announced in December they are expecting their first child.