Beneath the complex were more than two miles of tunnels which would have enabled slaves to move from the basement of one building to another without being seen by the emperor, his family and imperial dignitaries.

The newly-discovered underground passageway has been dubbed by archaeologists the Great Underground Road — in Italian the Strada Carrabile. Many of the tunnels have been known about for decades but this one is far larger than the rest. It was discovered after archaeologists working at the site stumbled upon a small hole in the ground, hidden by bushes and brambles, which led to the main gallery. Around 10ft wide, it runs in a north-easterly direction and then switches towards the south.

“All the majesty of the villa is reflected underground,” Vittoria Fresi, the archaeologist leading the research project, told Il Messagero newspaper.

“The underground network helps us to understand the structures that are above ground.” In contrast to the palace, which fell into disrepair after the fall of the Roman Empire and was plundered for its stone, the underground network remains “almost intact”.

The tunnel has been explored by a society of amateur archaeologists with caving and abseiling skills, as well as by wire-controlled robots equipped with cameras. Much of it is blocked by debris that has accumulated over the centuries. Heritage officials are hoping to organise the first public tours of the tunnels in the autumn.

“After a lot of work, we are preparing to open several areas to guided visits,” said Benedetta Adembri, the director of Hadrian’s Villa.

Hadrian, who built the eponymous defensive wall in northern England, was a keen amateur architect who incorporated into the design of his villa architectural styles that he had seen during his travels in Egypt and Greece. He started building the palace shortly after he became emperor in 117AD and continued adding to it until his death in 138AD. It included dining halls, fountains, and quarters for courtiers, slaves and the Praetorian Guard.”

By Nick Squires, Rome