Slabs of ancient plaster have fallen from the ceiling of the Colosseum, leading experts to call for a £20 million restoration of Italy's most famous Roman monument.

The three chunks of mortar plummeted to the ground around dawn on Sunday, a few hours before thousands of tourists tramped through the gladiatorial arena.

They crashed through a wire protection net which was supposed to have prevented such accidents, but which is more than 30 years old.

Archeologists warned that disaster had only narrowly been averted and that visitors could have been badly injured or even killed by the debris.

The plaster, which dates from Roman times, fell from a 10 square foot section of roof in one of the stone entrance ways through which spectators used to file to watch gladiators take on wild animals, prisoners-of-war and each other.