The new findings are believed to date back to the fifth century, and form part of the Great Palace of the Byzantine Empire.

Hagia Sophia: Site of excavations

Istanbul, then called Constantinople, was the capital of the Byzantine Empire, which ruled much of the known world from 476AD until the capital fell to the Ottomans in 1453.

However, aside from a few monuments, relatively little is known about the Byzantine city, and many of its archaeological glories remain underground and unexcavated.

The BBC's Chris Morris visits the excavation site

The archaeologists first discovered a hall believed to be from the Ottoman era. But a passage at the back of the hall led to a much older area that archaeologists believe formed part of the Great Palace.

On the wall were brightly-coloured frescos, still perfectly preserved after a thousand years.

The Director of Istanbul's Archaeological Museum, Alpay Pasinli, said the discovery was the result of seven months of careful excavations.

"As an archaeologist, of course, we feel very great satisfaction for this discovery. We may understand - after some study - some of the functions of the Great Palace," he said.

Archaeologists hope to exhibit some of the new finds as an open-air museum, but it may not be for many years yet.

Problems uncovering the past

Although the Byzantine remains are believed to be extensive, many of them remain buried below the thriving modern metropolis of Istanbul.

Byzantine artefact found in Istanbul

The Great Palace itself was a vast complex, containing buildings, courtyards and gardens.

But building and development in Istanbul have increased dramatically in the last few years, making widespread excavation almost impossible.