AT LEAST 300,000 Italians living near the Vesuvius volcano would be killed the next time it erupts if they are not evacuated beforehand, according to the first three-dimensional simulation by supercomputer.
However, up to 200,000 others living in the north-north-western areas of the high-risk "Red Zone" could have more time to escape thanks to the volcano's towering Mount Somma rim, which acts as a natural barrier, scientists say.
"For the first time, we have seen that these flows could be substantially diverted," Augusto Neri, of the National Geophysical and Vulcanology Institute in Pisa, who led the research, said.
"It seems Mount Somma acts as an effective barrier. But this doesn't mean they're safe."
Although Vesuvius has slept for more than six decades, scientists fear the next big eruption could rival the one in AD79, which buried the ancient cities of Pompeii and Herculaneum and killed about 16,000 people.
The authorities have a plan to evacuate the more than half a million civilians living in the 18 towns within a four-mile radius of Vesuvius. They estimate they can get everyone to safety within a week.
Mr Neri's research, published in Geophysical Research Letters, could give authorities a greater understanding of how the will behave in the event of a big eruption - and which communities need to be evacuated first. "We've already passed the civil protection authorities these results. They are going to consider some of the results of this simulation in the revision of their emergency plan," he said.
Peter Baxter, at Cambridge University's Institute of Public Health, who also participated in the study, said history looked set to repeat itself by pummelling modern-day communities around the ruins of Pompeii and Herculaneum.
"The pyroclastic flows are going to be pushed to the south-side, toward the sea," he said, adding "that's where Pompeii and Herculaneum are".
Mr Neri said the Vesuvius computer simulation was the first in 3-D that showed what a big eruption would look like over a period of time, factoring in complex variables such as topography.
The scientists specifically tried to simulate the effects of a collapse of the eruption column - what happens when the exploding mushroom cloud is too heavy for the air, and the material comes crashing down the sides of the volcano.
The crater sits some 4,200ft above and 13 miles away from the Naples, Italy's third largest city.
The simulation calculated the temperature of magma would be 950C - 1,742F - as it left the crater, and 200C at the outer edge of the Red Zone, four miles away.
• STROMBOLI, the Italian volcanic island popular with tourists, has started to erupt more violently than usual, sending a flow of lava into the sea, emergency services said yesterday, advising locals to keep away from the danger areas.
The cone-shaped island is an active volcano attracting thousands of visitors every year to climb to its summit and admire the regular blasts of molten rock, but more powerful eruptions can be dangerous. The last time, in December 2002, a large eruption caused the island to be closed for months.