A metal detectorist has unearthed a unique Anglo-Saxon silver penny which sheds light on the murder of a king 1,200 years ago.

Darrin Simpson, from Eastbourne was about to find shelter from a hail storm when his detector beeped and he found the coin six-to-eight inches below ground.

“I thought it was a Saxon coin, the first one I had found, and I was very happy about that, ” said Mr Simpson.

Mr Simpson is keeping the location of his discovery under wraps.

The 48-year-old contacted the Early Medieval Corpus of Coin Finds at the Fitzwilliam Museum in Cambridge and was told that the silver penny was the only one of its kind ever discovered.

Three others have been found from the reign of Æthelberht II, the late eighth century ruler of East Anglia, but this is the first to bear the title REX (“king”) on the same side as his name.

Mr Simpson, a pest control specialist, said: “The condition is really good. This is a unique coin – I doubt if I will ever find anything better.”

Æthelberht was king from 779 until 794 when Offa, the much more powerful king of neighbouring Mercia, invited him to his villa at Sutton Walls in Herefordshire to marry his daughter.

Offa had Æthelberht seized, bound and beheaded.

Until now there was no clue to his motive.

A spokesman for international coin and medal specialists Dix Noonan Webb said: “This find changes our knowledge of Anglo Saxon coinage. Saxon coins weren’t just used for day-to-day transactions, they were a way for rulers to project their image.

“If Offa thought Æthelberht was getting too big for his boots, that might be why he was so brutally murdered.”

According to legend, Æthelberht’s severed head later fell off a cart and rolled into a ditch.

After it was found it was said to have restored a blind man’s sight, resulting in the dead king being declared a saint.

The coin will be auctioned on June 11 at Dix Noonan Webb in London, when it is expected to fetch between £15,000 and £20,000.