The piece of cake was given to Moyra Smith, who worked at Clarence House for the Queen Mother, the auctioneer said, adding that this seemed to have been from one of the cakes sent to the staff.
The 40-year-old slice, which has been kept in plastic wrap in an old cake tin, was sold to Dominic Winter Auctioneers in 2008 on behalf of Smith’s family.
The auctioneer said it “appears to be in exactly the same good condition as when originally sold, but we advise against eating it.”
The auction is on August 11. The cake slice is expected to go for at least 200 pounds, or $278.