An ancient gold coin proves that a third-century Roman emperor, written off by history as a fictional character, really existed, according to scientists.

The coin bearing Sponsiano's name and portrait was found more than 300 years ago in Transylvania (modern Romania), once a distant outpost of the Roman empire.

It was thought to be a fake and was locked away in a museum cabinet.

Now scientists say scratch marks visible under a microscope prove it was in circulation 2,000 years ago.

Professor Paul Pearson of University College London, who led the research, told BBC News he was surprised by the discovery.

“What we found is an emperor. It was a figure that was considered a fake and had been deleted by the experts," he said.

"But we believe he was real and that he had a role in history."

What is true of all this?

The coin at the center of the story was among a small hoard discovered in 1713. It was considered a genuine Roman coin until the mid-19th century, when experts suspected it might have been produced by forgers of the time due to its crude design.

The final blow came in 1863, when Henry Cohen, the leading coin expert of the time at the Bibliothèque Nationale de France, addressed the problem for his large catalog of Roman coins. He said they were not only "modern" imitations, but poorly made and "ridiculously fantastic". Other experts agreed, and to this day Sponsiano has been dropped from scientific lists.

But Professor Pearson suspected otherwise when he saw photographs of the coin while researching a book on the history of the Roman Empire. He could make out scratches on its surface that he thought might have been created by the circulation of the coin.

He contacted the Hunterian Museum at the University of Glasgow, where the coin was kept locked in a cabinet along with three others from the original hoard, and asked to work with researchers there.

They examined all four coins under a powerful microscope and confirmed in the journal PLOS 1 that there were indeed scratches and that the patterns were consistent with them being carried in wallets.

A chemical analysis also showed the coins had been buried in the soil for hundreds of years, according to Jesper Ericsson, who is the museum's curator of coins and collaborated with Professor Pearson on the project.

Researchers must now answer the question, who was Sponsiano?

Researchers believe he was a military commander who was forced to commit suicide as emperor of the most remote and hard-to-defend province of the Roman empire, called Dacia.

Archaeological studies have established that Dacia was cut off from the rest of the Roman Empire around 260 AD. There was a pandemic, civil war, and the empire was fragmenting.

Surrounded by enemies and cut off from Rome, Sponsiano probably assumed supreme command during a period of chaos and civil war, protecting the military and civilian population of Dacia until order was restored and the province evacuated between AD 271 . and 275 AD, according to Jesper Ericsson.

"Our interpretation is that he was in charge to maintain control of the military and the civilian population because they were surrounded and completely cut off," he said. "In order to create a functioning economy in the province they decided to mint their own coins."

This theory would explain why the coins do not look like those of Rome

"They may not have known who the real emperor was because there was a civil war," says Professor Pearson.

“But what they needed was a supreme military commander in the absence of real authority from Rome. He took over management at a time when management was needed."

Once the researchers determined that the coins were authentic and that they had discovered what they believed to be a lost Roman emperor, they notified researchers at the Brukenthal Museum in Sibin, Transylvania, which also has a Sponsiano coin. It was part of the inheritance of Baron Samuel von Brukenthal, the Habsburg Governor of Transylvania. The baron was studying the coin at the time of his death, and the story goes that the last thing he did was write a note that said "genuine."

Experts at the Brukenthal museum had labeled their coin a historical imitation, as had everyone else. But they changed their minds when they saw the UK research.

The discovery is of particular interest for the history of Transylvania and Romania, according to the interim director of the Bruckenthal National Museum, Alexandru Constantin Chituță.

"For the history of Transylvania and Romania in particular, but also for the history of Europe in general, if these results are accepted by the scientific community, they will mean the addition of one more important historical figure to our history," he said.