Pompeii, preserved under ash from a volcanic eruption in 79AD and rediscovered in the 18th century, has been hit by a series of collapses in recent months and years that have sparked international outcry over the neglect of the site.
Officials said the wall of a tomb about 1.7m high and 3.5m long collapsed in the necropolis of Porta Nocera in the early hours of yesterday morning. It followed a smaller collapse on Saturday, when of part of an arch supporting the Temple of Venus fell away.
Heavy rains were cited as the immediate cause.
The Temple of Venus is in an area of the site that was already closed to visitors, while access to the necropolis has been closed following the collapse of the wall.
Culture Minister Dario Franceschini, appointed last month in the new government of Prime Minister Matteo Renzi, summoned officials responsible for the site to Rome for an "emergency meeting" on Tuesday.
He said he wanted a report on the reasons for the latest collapses and would verify routine maintenance at Pompeii as well as the progress of an ambitious restoration project launched last year with European Union funds.
Italian media have highlighted the contrast between the management of Pompeii and a successful exhibition about the ancient Roman city at the British Museum in London last year, which attracted record numbers of visitors.
Pompeii, a Unesco World heritage site, was home to about 13,000 people when it was buried under ash, pumice pebbles and dust as it endured the force of an eruption equivalent to 40 atomic bombs.
Two-thirds of the 66-hectare (165-acre) town has since been uncovered.
The site attracts more than two million tourists each year, making it one of Italy's most popular attractions.