Despite being immersed in seawater for more than 1,000 years, the plaque was glistening on the seabed and in almost pristine condition when it was found by divers working with Franck Goddio, the French archaeologist who is collaborating with the Centre for Maritime Archaeology at Oxford University.

Until his excavations in the Bay of Aboukir four years ago, Heracleion, the gateway to the land of the pharaohs, was known only from ancient texts, an inscribed stele found in the 19th century and the Stele of Thonis, found by Goddio in 2000, which names Heracleion by its Egyptian name, Thonis.

The importance of the latest discovery is that it confirms Thonis is the city the Greeks called Heracleion.

Heracleion, which has a similar name that of the ancient city on Crete, was Egypt’s main port and customs post long before Alexandria was founded in 331BC. It was famous throughout the ancient world. The Greek historian Diodorus recorded that Heracleion was named in gratitude for the Greek god Heracles, who stopped a Nile flood and saved the city.

Excavations have shown that the city possessed a magnificent temple dedicated to the supreme pharaonic deity, Amon, worshipped at Heracleion as Amon-Gereb, the god who gave the pharaohs their right to rule Egypt. The Ptolemaic rulers claimed to be his descendants.

According to legend, the Temple of Heracles-Khonsu was visited by Paris and Helen of Troy. The Greek historian Herodotus, who visited Egypt in about 450BC, tells how the lovers fled there to escape the fury of her jealous husband, Menelaos. But Thonis, the watchman at the mouth of the Nile, refused to help the adulterous couple.

Heracleion was destroyed by a series of natural disasters, including earthquakes, a possible flooding of the Nile, sea-level rise and a tidal wave, sending it crashing to the Mediterranean seabed about 1,200 years ago. The delta has so eroded that the site is now more than 6km (3¾ miles) from the coastline.

Greek inscriptions on the plaque, which measures 60cm by 10 cm, (26 in by 4 in) show that it was dedicated to Ptolemy III and his wife Berenice, as benefactors of an edifice or a god. The plaque, which will be housed in the Greco-Roman Museum in Alexandria, was found near a sacred ritual canal north of the temple. M Goddio said: “It is extremely exciting to find such an artefact in situ in that city.”

Jonathan Cole, an archaeologist and director of the Oxford Centre for Maritime Archaeology, said: “It’s such a dramatic find. Gold is rare enough, but to find an inscribed gold piece is quite astonishing.”

Only a small percentage of the sunken city has been excavated so far. It could take many decades to complete. A deep layer of silt has preserved carvings from toppled buildings and colossal statues including that of Hapi, the god of Nile fertility, as well as gold coins and bronze figurines.

The treasures reflect that “it was superbly honoured and maintained by the Ptolemaic kings”, M Goddio said.

Scholars from France, Germany, Egypt and elsewhere will visit Oxford University next month for an international archaeology conference on ancient Alexandria. The plaque will be presented among recent research.

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