Rare silver coin
The 1,200 year old coin had a guide price of £20,000 - but sold for £78,000 Dix Noonan Webb

A rare silver coin has sold for £78,000 at auction, and experts claim its discovery could provide a clue to the gruesome murder of an Anglo-Saxon king 1,200 years ago.

The coin was struck in the reign of East Anglian ruler Aethelbert II and describes him as king – the only time this title has been found on a coin from this era. Auctioneers said it was "miraculous" that the coin survived more than 1,200 years underground in such good condition.

Today the kingdom of Aethelbert II would straddle parts of Suffolk and Norfolk. Little is known of his reign, which may have begun in 779. Before this coin came up for auction in London this week, very few of the coins issued during his reign had been discovered.

It is thought that he was killed on the orders of Offa, the powerful king of Mercia, in 794. Historians speculate that the murder of Aethelbert II was ordered after he became too ambitious and threatened the reign of King Offa.

Some historic sources speculate that he was taken captive whilst visiting his future Mercian bride Aelfthyth and murdered.

According to legend, Aethelbert's severed head later fell off a cart and rolled into a ditch. After the bloodied head was found, it was said to have restored a blind man's sight. The "miracle" resulted in the dead king being canonised and declared a saint.

A spokesman for auctioneers Dix Noonan Webb told the Daily Mail that the coin had provided new insights into Anglo Saxon coins.

"Saxon coins weren't just used for day-to-day transactions, they were a way for rulers to project their image", he said. "If Offa thought Aethelbert was getting too big for his boots, that might be why he was so brutally murdered.

"This coin could easily have been destroyed by a plough or a digger or just a shovel. We don't know when it went into that field but it was probably well before the Battle of Hastings [in 1066]. It's miraculous that it has survived."

Darrin Simpson, who found the coin in a Sussex field in March using a metal detector, says he plans to split the money earned from the sale of the coin with the farmer on whose land he found it.

He said: "It's fantastic, an amazing result. I am really quite shocked."