The gold coin dated back from around 50 BC, is smaller than a fingernail and is stamped with the name Esunertos, a previously unrecorded King from the Iron Age.
Detectorist Lewis Fudge found the ancient currency which is being described by experts as “one of the outstanding discoveries of recent decades”.
He dug up his £20,000 find in a farmer’s field in March earlier this year after he got permission to detect the area.
Experts originally expected it to be purchased for around £4,000 at Spink, an auction house based in London.
However, a bidding war soon heated up as the auctioneers saw the price rise to £20,400, including premium.
This beats the world record for a similar type of coin, a ‘Medusa’ Quarter-Stater set at £10,800.
On the auction sale, Mr Fudge said: “I am over the moon, if it were not for people in the auction room I would have jumped around.
“The collectors I spoke to are gobsmacked. I’m so glad I did not take them up on their private offers before the auction. To think my find has generated its own Wikipedia page is incredible.”
Gregory Edmund, Iron Age Coin Specialist at Spink, led the team of experts in the recording of this new find.
He explained: “This fabulous piece of prehistoric artwork completes the mental image we have when we think of Iron Age Britain – the war horse and chariot.
“But it also surprises us with the appearance of classical languages like Latin.
“This is the reason I come to work; to document the discoveries of national importance and share that knowledge directly with museums and amongst academics, collectors and the public at large.”
The rare coin expert described the experience of finding the ancient currency as particularly “vindicating” to him.
Mr Edmund added: “ I focused my university degree on the Roman invasion of Britain through the lens of Britain’s first coinage.
“To now add a critically important contemporary witness to those seismic events in the birth of our island’s story is electrifying.
“Despite the coin’s diminutive size, the name of its conceiver – Esunertos – now truly echoes down the ages. Esunertos was once forgotten, but now his name looms large in the historic record.”