The pictures reportedly show Kate wearing a bikini and strolling on a beach on the island of Mustique with William. It is understood Chi magazine is planning to publish the images.
Last year the publication produced a 26-page spread showing Kate topless during a holiday in France.
A St James's Palace spokesman said: "We are disappointed that photographs of the Duke and Duchess on a private holiday look likely to be published overseas.
"This is a clear breach of the couple's right to privacy."
The pictures showing the Duchess' very slight baby bump are expected to hit the news stands tomorrow.
On a website publicising Chi - owned by former Italian prime minister Silvio Berlusconi - Kate is pictured on the magazine's front cover in a blue bikini striding knee-high through the sea above the headline "the belly grows".
A second image shows the Duke, wearing a pair of shorts, with his left arm around his wife as they walk along a beach.
It is believed that the royal couple are still holidaying on Mustique and during their time on the island were reportedly joined by Kate's family, parents Carole and Michael Middleton and her siblings James and Pippa.
A St James's Palace spokesman would not comment on whether William had been informed about the issue or if they were considering any form of legal action against the magazine.
The publication comes less than six months after the printing of pictures showing the Duchess wearing just a pair of bikini bottoms.
William was left visibly outraged by the intrusion into his wife's privacy while they stayed at a private estate in southern France last summer.
France's Closer magazine was the first publication to print the images in September last year and the Duke and Duchess later succeeded in getting an injunction from a French court stopping further pictures appearing in the title.
At the time Chi's editor Alfonso Signorini defended his decision to also publish the topless pictures saying: "I am a director of a newspaper not a supermarket, I don't sell artichokes and carrots, I sell photographic scoops."