The value of the hoard has not yet been determined and it is also unclear how the coins ended up where they were discovered.
They may have been used to mark the boundary of a property or as an offering to the Gods, archaeologists believe, in a woodland clearing or near a sacred spring.
It is not unusual for a hoard to be found in an apparently isolated location and they could also have been buried as savings or for an emergency in times of crisis.
The term ‘potin’ refers to the base metal silver-like alloy used in coins, which is typically a mixture of copper, tin and lead.
The first potins produced in Britain are known as Kentish Primary or Thurrock types, and are likely to have been made no later than 150 BC.
Sometime before 100 BC, these rather bulky coins were replaced by thinner coins with more degenerate designs, now called Flat Linear types.
Over a period of several decades, the Flat Linear potins gradually evolved into a wide variety of forms, with the depiction of the bull and the head of Apollo becoming more and more stylised. The Hillingdon Hoard is late in the Flat Linear sequence.
A hoard of a similar size, the ‘Sunbury hoard’ was discovered in 2010 but the potins were dated much earlier in the Iron Age. Potins from late in the Iron Age, similar to the Hillingdon Hoard, have been found previously but in much smaller quantities.
The latest potins were discovered following a storm, which caused ground conditions to change and revealed where they had been buried.
Emma Tetlow, now working for HS2’s main works contractor Skanska Costain STRABAG joint venture, said: ‘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.
‘The patch of soil was dark greeny-blue which suggests oxidized 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.’
Because of the significance of the find, and the high number of coins, the local coroner was alerted.
They will determine whether the discovery amounts to ‘treasure’ after considering specialist evidence from the British Museum.
The potins were taken to the Birmingham Museum and Art Gallery where they have been cleaned and preserved.
HS2’s head of heritage Helen Wass said: ‘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.
‘HS2’s unprecedented archaeological programme has enabled us to tell the stories of our history and leave a lasting legacy for future generations.’
Earlier this year HS2 archaeologists also unearthed the remnants of stunning gardens belonging to a 16th-century manor house.
The discovery, near Coleshill on the outskirts of Birmingham, was dubbed ‘Warwickshire’s answer to Hampton Court’.