The pictures show the pregnant Royal wearing a bikini while strolling along a beach on the Caribbean island of Mustique, during a private break with husband William.
They show the Duchess' very slight baby bump.
Chi magazine, which last year produced a 26-page spread showing Kate topless on holiday in France, will publish the new images on Wednesday.
"We are disappointed that photographs of the Duke and Duchess on a private holiday look likely to be published overseas," a St James's Palace spokesman said.
"This is a clear breach of the couple's right to privacy."
It is believed that the royal couple are still holidaying on Mustique where they reportedly joined Kate's family, parents Carole and Michael Middleton, brother James and sister Pippa.
They are expecting their first child in July.