The Kate Middleton brand name has been protected the royals warn shops.

Fashion retailers can be sued for ten-thousands of pounds if they suggest that Kate Middleton in any way is promoting their clothing, courtiers have warned.

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A legal expert told the Daily Mail: “Her name is now controlled by the Royal Family. She has a reputation to uphold and because of that she is, to all intents and purposes, akin to a brand or marque.”

But what can retailers do to avoid prosecution?

A royal aide explained: “(It is) highly unlikely that we would have any objection if a company wanted to call a dress ‘the Kate dress’.

“‘Kate’ isn’t specific to Catherine Middleton. It may, however, be a different scenario if it was called the ‘Catherine/Kate Middleton’ dress as that impacts … on personal copyright. Or indeed if a company was suggesting a dress was officially endorsed by Catherine.”

Already dubbed a style icon, Kate Middleton’s £399 blue Issa dress that she wore to the official engagement announcement sold out within a day.

Kate Middleton celebrated her last birthday as a “commoner” yesterday. The 29th birthday party was kept firmly under wraps, with Clarence House revealing only that she as was spending the day “privately”.