Cake designer Fiona Cairns poses with the royal wedding cake. Photo: Getty

A three-year-old slice from Prince William and Kate Middleton’s wedding cake could raise as much as $US2000 ($A2163) next month when it is put up for auction in California.

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Julien’s Auctions in Beverly Hills, California is offering the royal slice at a sale in December dedicated to items belonging to William’s mother, the late Princess Diana.

The fruit-filled cake from the 2011 wedding will be sold wrapped in white paper in a decorative box and with a blank invitation from the event.

The cake will be featured in the California auction alongside five dresses that belonged to Diana.

The most expensive dress, an ivory-colored silk evening dress by Zandra Rhodes, is valued at between $US80,000 and $US100,000.

Next month’s auction takes place in Beverly Hills from December 5-6.