An Anglo Saxon coin which is the only one of its type to be discovered is expected to fetch between £15,000 and £20,000 at auction.
The 1,200-year-old silver penny, minted during the reign of Aethelberht II, was found by Darrin Simpson, 48, from Eastbourne, East Sussex.
The pest control specialist was sheltering from a hailstorm in a Sussex field in March when he picked up a signal on his metal detector and dug down six to eight inches, finding the coin, a spokesman for the auctioneer said.
Mr Simpson said he was expecting it to be a Second World War era .303 bullet case but was happy to discover the coin.
It was not until he contacted the Early Medieval Corpus of Coin Finds at the Fitzwilliam Museum in Cambridge that he realised the full importance of his discovery.
He said: "It was a bit of a shock really, I couldn't sleep for two nights after it was identified.
"The condition is really good. This is a unique coin. I doubt if I will ever find anything better."
The coin is only the fourth ever found from the reign of Aethelberht II, a shadowy figure who ruled East Anglia in the late eighth-century. The other three are all in museums and have a different design, according to Christopher Webb, head of the coins department at Dix Noonan Webb.
This declaration of independence by the East Anglian king could provide a clue as to why he was murdered by a neighbouring monarch, Mr Webb said.
The coin will be auctioned at Dix Noonan Webb, the international coins and medals specialists, in London on June 11.