Specialists at the Brukenthal museum kept the coin locked away in a cupboard thinking it was a fake, but changed their minds when they saw the research led by Professor Paul Pearson at University College London.

The coin bears the name “Sponsian”, that of the forgotten emperor, along with his portrait. The coin was found some 300 years ago in modern-day Transylvania, where there was once a Roman outpost.

Microsopic scratch marks or grooves helped prove the legitimacy of the artefact, showing that it was in circulation 2,000 years ago.

Professor Pearson told BBC News: “What we have found is an emperor. He was a figure thought to have been a fake and written off by the experts.

“But we think he was real and that he had a role in history.”