Experts said it was made a few years after Actium but was a terrible fake, probably created from memory by a craftsman who was ‘barely literate’.
One face has a crocodile but it is facing the opposite way to the original.
Emperor Caesar is on the head side, when it should have been Augustus.
And the die cutter misspelled Egypt as Aegipto instead of the common spelling of the time, Aegypto or Aegvpto.
Cleaner Rob Clements, 45, discovered the coin buried two inches beneath the surface on a grass path near Brighton, East Sussex, just months after buying his first metal detector.
Sam Moorhead, national finds adviser for Ancient Coins at the British Museum, said nothing like it had ever been seen before, pushing up its value from £100 for the average Roman coin to about £3,000.
Mr Moorhead said he was mystified by the motive of the forger, who could not even have made a profit because he struck it from solid silver.