An Ancient Roman staircase which appears to have led into a previously unknown major building has been found during excavations for a new subway station.

Archaeologists immediately dubbed the white-marble staircase, the latest in a trove of finds at the site, ''the imperial steps''.

Only a part of the staircase - five steps measuring some ten metres - has so far been uncovered.

It is inset into pink granite and the Romans' favourite monumental building stone, travertine.

''This is an extremely important find and completely unexpected because the staircase was not known,'' said Rome Archaeological Superintendent Angelo Bottini.

''It must have been an entrance into an important place but we have to find something in ancient sources if we are to make any circumstantiated hypothesis,'' he said.

''At the moment we can't even make a precise dating''.

Bottini said there was no trace of the monument ''even in the Forma Urbis,'' a massive marble map of ancient Rome created under the emperor Septimius Severus between 203 and 211.

The map was destroyed in the Middle Ages but much of its content is known from ancient writings. Archaeologists have managed to assemble shards of some 10% while Stanford University experts are using computer algorithms to try to recover more of it digitally.

The remains of brickwork pillars, which archaeologists say may have collapsed in an earthquake ''in ancient times'', were found alongside the stairs. The staircase was discovered just around the corner from the Ancient Forum in the middle of Piazza Venezia, the central Roman square where Benito Mussolini gave his speeches.

One of the 30 stations on Rome's new C metro line, the third in the capital, is being built in the square.

All the Roman, medieval and Renaissance artefacts and monuments which digs have thrown up will be showcased in the future station.

Roman taverns, a sixth-century copper factory, medieval kitchens still stocked with pots and pans and remains of Renaissance palaces are among the finds that have turned up over the last ten months Bits of the ancient Via Lata, one of the main roads out of the city, have also been found.

The Via Lata was what Via Flaminia - the most important highway to northern Italy - became once it entered the city.

Today's Via del Corso follows its course from Piazza Venezia to Piazza Del Popolo.