Stephen Eldridge found the 12 Iron Age pieces while searching farmland in Buckinghamshire.
Experts at the British Museum identified them as originating from a tribe in what is now Picardy in France and made in 150BC.
It is thought that the coins would have been exported to Britain probably in exchange for Celtic mercenaries going to Gaul in western Europe to fight the Romans.
While individual gold coins of this period have been found before, a hoard from this date is incredibly rare.
A metal detectorist has uncovered Britain’s oldest hoard of gold coins dating back 2,173 years
Stephen Eldridge found the 12 Iron Age pieces while searching farmland in Buckinghamshire
The coins are now expected to sell for £30,000 when they go under the hammer at London auctioneers Spink & Son.
Mr Eldridge, 68, found the coins in the village of Ashley Green, Buckinghamshire, in November 2019.
In 150BC, the area was inhabited by the Catuvellauni tribe, which rose to become the most powerful tribe in Britain over the next century.
After going through the treasure process Mr Eldridge has now put the coins up for auction with London-based coin specialists Spink.
Scientific x-ray fluorescence analysis confirmed the coins to be approximately 75 per cent gold with an alloy of silver and copper, pointing to the economy in which Britain’s first gold coins circulated.
Gregory Edmund, of Spink & Son, said: ‘Whilst individual gold coins of this period have been recorded across south east England, it is incredibly rare for a trove of this size or date to be uncovered.
‘Contemporary local coinage was simply cast base metal issues called ‘potins’. Whoever successfully imported this trove of gold coins would have undoubtedly wielded influence in the region.
Experts at the British Museum identified them as originating from a tribe in what is now Picardy in France and made in 150BC
‘They would have been exported, probably in exchange for mercenaries, equipment and hunting dogs to fight the Romans or other tribes in Belgium.
‘Twenty or thirty years after they were deposited we started to get the first British coins in the same style.
‘These coins were in the wealthiest part of the English kingdom.
‘A hoard of this size and period is unprecedented in the archaeological record.
‘There was one other hoard from this period of three coins found.
‘These coins have been well used, it is very clear they are not fresh when they are put in the ground, but still retain remarkable details of a seldom-seen Iron Age art form.
After going through the treasure process Mr Eldridge has now put the coins up for auction with London-based coin specialists Spink
Scientific x-ray fluorescence analysis confirmed the coins to be approximately 75 per cent gold with an alloy of silver and copper, pointing to the economy in which Britain’s first gold coins circulated
‘It is often speculated that the portraiture of this coinage was deliberately androgynous despite being modelled on the classical male god Apollo.
‘The feminine styling is probably a reflection of the political significance of women in Iron Age society, that enabled such historical figures as Cartimandua and Boudicca to rise to prominence and our now national folklore.
‘It is incredibly satisfying to assist in the proper recording, academic analysis and now sale of these prestigious prehistoric relics.’
The British Museum elected to disclaim the coins after coroner’s inquest, meaning they pass back to the finder.
Mr Eldridge will share the proceeds with the landowner.
The coins will be sold at Spink on Thursday.
