The monumental building was probably constructed under Emperor Justinian I in the first half of the 6th century.
The floor plan and recognisable architecture of the basilica show striking parallels to the Eastern Roman Empire - which makes the discovery particularly exciting, ÖAW archaeologist Stefan Groh told the Viennese news agency Kathpress on Wednesday.
According to archaeologists, this is the first newly discovered large building in decades in the already intensively researched city. Aquileia was once a metropolis of the Roman Empire and an ecclesiastical centre with a patriarch until the Middle Ages.
Located around ten kilometres from the Grado lagoon, the city is known for its cathedral with the largest preserved early Christian mosaics in the western world. In 1998, Aquileia was declared a World Heritage Site by Unesco.
The site where the researchers made the ancient discovery lies to the west of modern-day Aquileia on the Via Annia, the trade route to Rome at the time.
Expansion into a basilica probably under Emperor Justinian
Geophysical measurements and boreholes have so far made it possible to identify the basilica without excavations.
Originally built in the 4th century as a simple church building, it was probably expanded into a three-nave transept basilica with three apses in the first half of the 6th century under Emperor Justinian, according to the Austrian Academy of Sciences.
This was a completely unique type of building that only existed in the eastern Mediterranean region at that time, i.e. in the Byzantine Empire, it said. The design is reminiscent of Byzantine domed architecture, such as that of the Hagia Sophia.
The architectural orientation towards the south-east in the direction of Constantinople and Jerusalem points to the reconquest of Upper Italy by Justinian.
"This building is not just a religious monument, but a sign of power and cultural integration in the region," said Groh. The find points to a larger Byzantine building programme.
The data collected on the discovery of the new basilica in Aquileia is to be published in specialist journals together with considerations on the chronology and genesis of the building.
In addition to authorisation from the authorities in Italy, funding would also be required for excavations as a further step, said Groh.
"Of course, we would have to excavate a small area to verify this and, above all, to reconstruct the chronological development of this huge, large church building."