Gold jewellery found on board a wrecked Byzantine-era ship shows that one of the people on board was likely a high-ranking person, archaeologists say.

The ship was discovered more than a decade ago off Croatia’s southern coast and has been subject to a years-long underwater excavation.

Archaeologists announced the results of their studies for the first time on Thursday, saying that the shipwreck could date to the 7th or 8th century.

The treasures on board showed that it was no ordinary merchant vessel, and it was probably carrying someone of high rank, along with their entourage, they said.

Among the haul were gold coins dating to the Heraclius dynasty of four emperors, which ruled the Byzantine Empire between 610 and 711 AD, along with buckles decorated with rubies, emeralds and pearls.

Archaeologists also found a golden signet ring with a picture of the emperor, which “certainly wouldn't be worn by just anyone”, Pavle Dugonjic, head of the Department of Underwater Archaeology at the Croatian Conservation Institute, said.

His team, equipped with diving gear, has spent the past decade slowly uncovering the history of the shipwreck, which lies near the Adriatic island of Mljet.

“This is an impressive amount of gold, the largest ever found on a shipwreck in the entire Mediterranean,” said Igor Miholjek, the head of research at the Croatian Conservation Institute.

“It weighs over 600 grams when weighed after cleaning and restoration.”

The Byzantine Empire was the successor to the Roman Empire and centred on Constantinople, which is now Istanbul in Turkey.

It once dominated the eastern Mediterranean, but by the early 8th century it was mired in turmoil and was losing territory.

The empire ended with the fall of Constantinople to the Ottoman Empire in 1453.

"For me, it's the most important shipwreck of its period," said Justin Leidwanger, professor of archaeology at Stanford University.

"If you're studying the end of the late Roman Empire and the transformation into the medieval world, this is a critical site for us to understand and to gain insights into how the Mediterranean was held together by its sea routes, how it fragmented, how the empire fell apart."