The commemorative slice is going up for auction, 40 years after it was dished out at the royal wedding.
It is expected to fetch up to £500 when it goes under the hammer on 11 August.
The large slice, from one of the 23 official wedding cakes, features the royal coat-of-arms coloured in gold, red, blue, and silver.
As part of the purchase, the buyer will also get an order of service, ceremonial details and a royal wedding breakfast programme.
The cake was given to Moyra Smith, a member of the Queen Mother’s household at Clarence House, who preserved the topping with cling film.
“It appears to be in exactly the same good condition, but we advise against eating it,” said Chris Albury from Gloucestershire’s Dominic Winter Auctioneers.
Mrs Smith kept it in an old floral cake tin and taped a handmade label to the lid, reading: “Handle with Care - Prince Charles & Princess Diane’s [sic] Wedding Cake,” which she signed and dated 27 July 1981.
Her family sold the cake to a collector in 2008 and it has now come up for auction 40 years after Charles and Diana were married.
Charles and Diana married at St Paul’s Cathedral on 29 July, 1981. The couple separated 11 years later in 1992, and divorced in 1996.