The marzipan-based cake slice is covered in white icing with a red, blue, and silver sugared design of the royal coat of arms. The cake slice also has a silver horseshoe and a metallic overlay.
Dominic Winter Auctionssays on its website that the cake icing layer was given to Moyra Smith, an employee to the Queen Mother at Clarence House.
"Moyra began her career at Clarence House in the kitchen before moving on to more general duties on the recommendation of Lady Jean Rankin. Besides the main official five-tier wedding cake, some 22 other wedding cakes were supplied by various manufacturers and this seems likely to have been one sent to Clarence House for the consumption of the Queen Mother's staff," the auction house continued on its website.
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Dominic Winter Auctioneers' website says that it acquired the cake slice in August 2008 after it was sold on behalf of Moyra's family.
"It appears to be in exactly the same good condition as when originally sold, but we advise against eating it," the company added on the same webpage.
Along with the cake slice, the auction house is also auctioning off printed materials, including the order of the royal wedding, ceremonial details, and a breakfast program.
The auction is scheduled for August 9 and 10. The items are expected to sell between about $278 to $417 (£200 to £300).
A representative for Dominic Winter Auctioneers did not immediately respond to Insider's request for comment. A representative for Clarence House did not immediately respond to Insider's request for comment.
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