Duchess Catherine on her trip to Australia

Britain's Duchess of Cambridge is reportedly terrified that a picture of her bare bottom is going to be leaked on the Internet.

A photographer claims to have taken the photograph while Duchess Catherine, 32, and her husband Prince William were on a royal tour in Australia recently and is said to be offering it to the highest bidder.

A source told the Daily Star newspaper: "Poor Kate. Even though, for once, it's her bum everyone is talking about and not her sister Pippa's, it would cause huge embarrassment and anger if this photo got out."

The photographer claims he took the snap when the royal couple arrived at the Blue Mountains in New South Wales by helicopter, and a gust of wind blew the Duchess' dress up momentarily.

The photo has already been offered to a number of magazines in Australia including Australia's Women's Weekly, who have written: "It's yet another paparazzi photo to surface of the Duchess which invades her privacy."

In 2012 a French magazine printed a picture of Catherine sunbathing topless while on holiday with William and the couple blasted the invasion of privacy as "grotesque and totally unjustifiable".