Britain's Duchess Catherine has reportedly turned down an opportunity to appear on the front cover of US Vogue.

The 29-year-old royal - who married Prince William in April - was asked by the prestigious magazine's editor Anna Wintour to appear in a spread but has declined the offer because she doesn't want to be seen as a fashion icon.

A source told E! News: "Both William and Kate feel it would be wrong for Kate to promote herself as a fashion or style icon. That's not what they are about and they take their royal duties far too seriously for that."

Catherine was also said to be wary of upsetting William's grandmother, Queen Elizabeth, as she knows the monarch wouldn't have been too keen on the idea.

The source continued: "Kate is keenly conscious that while her new grandmother-in-law may not chide her for appearing on a Vogue cover. But she would definitely not be amused, and Her Majesty is a real genius at making people squirm with simply a look or a 'tut, tut.'

"To be brutally honest, a request to sit down with The Times, the BBC or one of the major American networks to discuss the monarchy in general is one thing. For Kate to be photographed draped in the latest creation of a couture house to, knowingly or otherwise, self-promote her status as some sort of fashion horse is something quite different."