London's building boom has given archaeologists an unexpected bonus -- the city's ancient past is being laid bare.

The latest piece of the historical jigsaw is most of the interior decor of a rich merchant's dining room dating back to 120 AD when the Roman Emperor Hadrian ruled an Empire stretching from northern England to northern Africa.

The decorated plaster was discovered under the floor of an Italian delicatessen on the edge of Leadenhall Market which in turn is next to the site of what was the city's Roman town hall.

One section of the green, blue and terracotta colored murals painted on plaster show a girl's head, a bunch of grapes and candelabra.

"This is an amazing discovery because it allows us to reconstruct the decoration within a Roman London room from the early second century," said Museum of London archaeologist Sophie Jackson.

"It is incredibly rare to have this much decorated plaster of such high artistic quality," she added at a press preview on Tuesday. "It must have been from the dining room of a very wealthy merchant or senior official living in the city centre."

The archaeologists, who were astounded not just by the quality but the sheer quantity of their discovery -- in all 45 crates full of Roman plaster have been removed from the site -- believe the house was destroyed by fire.

"There were scorch marks on some of the plaster, and we know that much of the city centre was destroyed by fire in 125 AD," Jackson told Reuters. "It is in such a good state of repair because the house was simply flattened and covered over."

Hadrian, better known for the remains of the wall he ordered to be built to keep the marauding Picts out of northern England, ruled the Roman Empire from 117 to 138 AD.

The Leadenhall discovery was seven meters down and covered by a protective layer of soil. Just next door, the Roman remains have gone, dug out centuries ago to make way for a medieval cellar.

But such is the boom in office building in the city that it is just one of about 50 sites being investigated at present.

Under planning rules, all building excavations must be preceded by an archaeological investigation to see what lies beneath.

The Romans occupied London from about 50 AD.

"The Romans were very good at erasing everything that went before them -- so there is very little pre-Roman to find in the city," said Jackson. "But from then on buildings were simply put on top of each other."

"London is essentially sitting on a 2,000-year-old rubbish dump, and there is so much left to find," she added. "We will have our work cut out over the next couple of years."