Britain’s Duke and Duchess of Cambridge are said to be upset after photographers captured the pregnant royal baring her baby bump on a private holiday.

Prince William and his wife Catherine Middleton recently enjoyed what they thought would be a romantic getaway on the island of Mustique.

Paparazzi followed the couple there.

They are said to have captured images of Catherine revealing a slight baby bump as she swam in a blue bikini.

A source told Us Weekly the photos have become “a real sour note”.

The Duchess is said to be mortified by how little control she has over the context of how her baby bump will first be revealed to the masses.

“They thought with the privacy laws on the island they would be able to spend a moment outside of the house without this happening,” the source said.

Catherine made a public appearance in London earlier this month, but kept her blossoming stomach hidden under a cape.

A spokesperson at St. James’s Palace issued a statement slamming the invasion of privacy.

“(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 statement read.

The pictures will be published in the next issue of Italian magazine Chi scheduled to hit stands on 13 February.

The publication ran topless photos of Catherine in September 2012.

The Palace has not confirmed if legal action will be taken.

Catherine is expecting her first child with Prince William in July.