The hidden ruins of an ancient lagoon city that was the ancestor of Venice have been unearthed by scientists using satellite imaging. The outlines are clearly visible about three feet below the earth in what is now open countryside.

Venice was a powerful maritime power during the Middle Ages and the Renaissance. It seemed, however, an unlikely spot to choose for a leading world power, stretching across 118 small islands in the marshy saltwater Venetian lagoon.

Historians agree that the explanation is that Venice was founded on the islands by refugees from Roman cities such as Ravenna, Padua and Aquileia as they fled from invasions, first by Attila the Hun in the 5th century and then, a century later, by the Lombards, as the final remnants of the Roman Empire crumbled.

However, Paolo Mozzi, a researcher at the University of Padua geography department, said high-definition satellite photographs had revealed the ruins of an extensive town much closer to present day Venice at Altino – known in Roman times as Altinum – a little more than seven miles north of the city, close to Marco Polo airport.

“The hypothesis is that as Altinum also succumbed to the Barbarian invasions, the inhabitants fled farther down to the lagoon to build Venice on the islands, using some of the stones from their city,” he said. At its height, he said, Altinum had been an important trading and seafaring centre on the Adriatic, before it was overrun by Attila in the mid-5th century.

The newly identified ruins include streets, palaces, temples, squares and theatres, as well as a large amphitheatre and canals, showing that Altinum was a wealthy and thriving city. By the 10th century, however, it had been abandoned. “The city in which Venice had its origins finally has a face,” said the Italian newspaper Corriere della Sera.

Mr Mozzi said that Altinum probably had about 20,000 inhabitants. A plan to excavate the ruins is being drawn up at the universities of Padua and Venice, in collaboration with the Veneto region superintendent of archaeology.

The folk memory of Altinum is preserved in the names of several Venetian islands which are derived from districts of the abandoned Roman town: Torcello (from Torricellum), Murano (Ammurianum) and Burano (Porta Boreana). In the 9th-century the ducal seat of Venice was moved to Rialto island, which is still the city centre, where the Doge’s Palace and the Basilica of St Mark were erected. In 828 the relics of St Mark were taken to Venice from Alexandria and placed in the new basilica. They were later venerated as a symbol of the new Venice as it developed into a mighty naval and commercial power in the Middle Ages.