The fruit cake has been perfectly preserved since the wedding reception held at Buckingham Palace to celebrate the marriage of Princess Elizabeth and Philip Mountbatten in 1947.
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Prince Philip cut the lavish four-tier cake — which stood at 9ft tall — with his ceremonial sword, before it was divided between the 2,000 guests. A layer of the wedding cake was kept aside for the christening of Prince Charles, a year later.
The slice that went under the hammer originally belonged to one of the newlyweds' guards of honour at their Westminster Abbey wedding ceremony.
The piece of cake has sold for £1,750
It was later donated to the Princess Alice Hospice in Esher, where it lay in a filing cabinet for 15 years until it was found during a sort out of the archives, the Telegraph reports.
A celebrity antiques dealer bought the 3.5 inch by 7.5 inch slice on Channel 4's Four Rooms in 2011 and he has now sold it at Christie's in London.
It eventually went under the hammer for £1,500 — but added fees brought the price up to £1,750. The cake was sold with its original Buckingham Palace case, which has been dated November 20, 1947.
Queen Elizabeth and Prince Philip's wedding cake
Charlotte George, from Christie's, said, "The cake was made using rum and brandy, which would explain how it has lasted so well.
"Remarkably, it is still in one piece, wrapped in baking parchment. I wouldn't recommend eating it — but as a collector's item, it is fantastic."