The slice, which measures eight inches by seven inches, has been kept wrapped up and inside an old cake tin for four decades. It features a detailed Royal coat of arms decoration on the icing and marzipan, painted in red, gold and blue.
Following the wedding ceremony in 1981, the slice was given to Moyra Smith, an employee of the Queen Mother’s at Clarence House.
Her family kept the cake until 2008. After that, a collector acquired it and is now selling it with auctioneers Dominic Winter in Cirencester, Gloucestershire.
A total of 23 cakes were made for Charles and Diana’s wedding, including a five-foot tall centrepiece fruitcake that weighed 225lbs (approximately 102kg).
The slice given to Smith was believed to have been cut from a cake that was then distributed to staff at Clarence House. Auctioneers have tipped it to fetch £500 when it is sold together with an order of service for the wedding, which took place at St Paul’s Cathedral, and a Royal Wedding Breakfast programme.
Chris Albury, a specialist valuer of royal memorabilia at Dominic Winter, said: “I still wouldn’t recommend eating it but after 40 years it’s clearly destined to last.
“It’s a curious and unique keepsake celebrating a royal wedding that holds an enduring fascination with British royalty aficionados worldwide.”
It comes ahead of what would have been Charles and Diana’s 40th wedding anniversary on 29 July. The pair separated in 1992 and divorced in 1996 after 15 years of marriage.
The sale of the wedding cake slice takes place on 11 August.
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