Hundreds of rare coins from the Iron Age have been uncovered by archaeologists working on the HS2 route.
One excavator described the hoard of small coins, which were found in Hillingdon, west London, and date back to the first century BC, as a “once-in-a-lifetime find”.
Under British law, the find could be recognised as “treasure” and acquired by a museum.
Bearing the side profile of Greek god Apollo on one side and a charging bull on the other, their design is based on coins struck in the French city of Marseille more than 2,000 years ago.
Excavation teams made the discovery after a storm disturbed the ground where they were buried.
Emma Tetlow, the historic environment lead for HS2’s main contractors Skanska, Costain and Strabag, described the moment the team uncovered the small coins, known as “potins”.
“We were coming to the end of our archaeological work on the site when we found a patch of soil that was a very different colour from what it would be expected to be,” she said.
“The patch of soil was dark greeny-blue, which suggests oxidised metal, and when we checked more closely, we could see loosely packed metal discs.
“This is a once-in-a-lifetime find and allows us to expand our knowledge of what life could have been like in Hillingdon many centuries ago.”
The coins, which are around 3cm in diameter, date back to when the Romans began establishing themselves in Britain and creating Londinium, and they circulated in Kent, Essex, Hertfordshire and Europe.
Historians believe they were used as an offering to the gods or to mark the boundary of a property, as bartering rather than currency was the main form of exchanging goods and services at the time.
Hoards are often found in isolated locations after being buried as emergency savings – though it is unusual to find one of this size.
Helen Wass, HS2’s head of heritage, said the discovery could give historians an insight into how people in London lived centuries ago.
“This is an exciting find for our team of archaeologists and provides us with more information about how our ancestors lived and settled in London,” she said.
“HS2’s unprecedented archaeological programme has enabled us to tell the stories of our history and leave a lasting legacy for future generations.”
Duncan Wilson, chief executive of Historic England, said the organisation is working with HS2 to ensure that any finds are surveyed and investigated.
He said: “They say that money talks, so what can this unexpected discovery tell us?
“It takes us back to the tumultuous time of Julius Caesar’s conquest of Gaul.
“Reaching Britain in 55 and 54 BC, he observed the locals using ‘bronze’ coins, an innovation they had adopted not long before through cross-channel migration and trade.”
A coroner will decide whether the coins count as “treasure”, meaning they can be acquired by a museum and their value determined – though the archaeologists will not be able to claim any monetary reward.
The potins have been cleaned and preserved at the Birmingham Museum and Art Gallery before specialists investigate their origins and significance further.
HS2 has also uncovered 11,000-year-old stone tools and artefacts in London, which suggested that people were living on the banks of the river during the Middle Stone Age.