The figures have been updated from a previous estimate, made in 2014, for the potential consequences of a huge earthquake along the Nankai Trough south of Japan.

The 800-kilometre (500-mile) undersea trench runs from Shizuoka, west of Tokyo, to the southern tip of Kyushu island.

It is where the Philippine Sea oceanic tectonic plate is "subducting" -- or slowly slipping -- underneath the continental plate that Japan sits atop.

The plates become stuck as they move, storing up vast amounts of energy that is released when they break free, causing potentially massive earthquakes.

The Cabinet Office's disaster management working group said up to 215,000 people would be killed by a tsunami, 73,000 by the collapse of buildings and 9,000 by fire.

But the predicted toll as a whole is lower than the 2014 estimate which said that up to 323,000 people would die.

Over the past 1,400 years, megaquakes in the Nankai Trough have occurred every 100 to 200 years. The last one happened in 1946.

Scientists say it is extremely difficult to predict quakes.

But in January, a government panel said the probability of such a megaquake in the next 30 years has marginally increased, with a 75-82 percent chance of it happening.

Last August, the Japan Meteorological Agency (JMA) issued its first "megaquake advisory" under rules drawn up after the devastating 2011 earthquake, tsunami and Fukushima nuclear disaster.

It said that the likelihood of a new major earthquake along the Nankai Trough was higher than normal after a magnitude 7.1 jolt in southern Japan which injured 14 people.

The advisory was lifted after a week but caused shortages of rice and other staples as people replenished their emergency stocks.