The coin, unearthed by a metal detectorist, is from a forgotten period of British history known as 'the Anarchy', during in-fighting between the grandchildren of William the Conqueror.
It was discovered in a Yorkshire field on March 12, dating from around AD 1148 to 1152 when a civil war raged over the right to rule England.
During this time of turmoil, a brief window in crown authority allowed local noblemen to start issuing coins in their own likeness, rather than the king.
As a result of this time of chaos, the coin was minted depicting an obscure Yorkshire Baron rather than the ruling King Stephen.
The coin shows a medieval knight on horseback brandishing a sword and is attributed to the obscure Baron Robert de Stuteville who was present at the ‘Battle of the Standard’, one of the more famous skirmishes at Cowton Moor near Northallerton on August 22, 1138.
Experts said the discovery is the first new find of its type to be documented in over 100 years and exect the coin to sell for at least £6,000.
Gregory Edmund, auctioneer for Spink and Son, said it was an "understatement" to just describe this coin as rare.
He said: "Although the past few months have truly tested us all, it has been a genuine pleasure to escape into the history of this remarkable new find and explore its fascinating legacy.
"It is an understatement to describe this coin as simply 'rare', for it is the first new find of its type to be documented in over a century, despite the coin being known since 1684 thanks to a chance discovery in a mole hill.
"Since then speculation has been wide ranging about its identity, including suggestions about Robert, Duke of Normandy, the eldest son of the Conqueror and even Robert, Duke of Gloucester, the illegitimate son of King Henry I.
"However, thanks to a re-reading of the obverse legend at the turn of the last Century, and as now confirmed on this new coin, the mysterious inscription clearly reads ‘RODBERTVS D STV’ or ‘Rodbertus de Stutavilla’ – the obscure Baron at the Battle of the Standard."
Edmund said the coin sheds light on an overlooked and forgotten part of British history.
He added: "As such, this is only the fifth coin of Robert de Stuteville to be authenticated, but the only one available to collectors as all the other coins are retained in institutional collections - Namely the Hunterian Museum (Glasgow); the National Museum of Wales (Cardiff); the British Museum (London); and the Fitzwilliam Museum (Cambridge).
"It is fantastic to shine a light on an overlooked and forgotten part of this country’s rich and troubled history, and to weave the latest great discovery into the diverse tapestry of the City of York already so well known around the world for its Roman and Viking heritage.
"It is also perhaps particularly apt at a time where we consider the very future of physical money, most of us having spent the past few months opting for only digital transactions that a find of this importance should come to light now."
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