The royal couple’s lavish cake consisted of four tiers and was laced with alcohol for the wedding on 20 November 1947.
Mr Grinter, Reeman Dansie’s royal expert, said Ms Polson’s slice was the first piece ever sold “in its completeness”.
He told Essex: “This one actually has its original contents which is very, very rare.
“Bear in mind it was produced at a time of rationing… They had the most magnificent cake made for them.
“I’ve seen photographs of it – it would fill half a room, it was absolutely enormous.”
Mr Grinter said the cake was no longer in the best condition, adding: “I don’t think I’d particularly want to eat it, I must admit.”
