A row of portable toiliets. A dreaded major earthquake in Tokyo would set off a crisis of "toilet refugees," with a restroom shortage for nearly 820,000 people, a government study has said.

A dreaded major earthquake in Tokyo would set off a crisis of "toilet refugees," with a restroom shortage for nearly 820,000 people, a government study said.

An expert panel of the government's Central Disaster Prevention Council came to the conclusion this week while studying the potential impact of a 7.3-magnitude earthquake in the tremor-prone metropolis.

According to the study, some 817,000 people would find themselves without toilets two hours after such a big tremor, which would cut off 46 percent of Tokyo's water supply.

The wait to use a toilet would be four and a half hours in central Chiyoda ward, home to the headquarters of major companies, government buildings and the imperial palace.

"Besides food and water, shortage of toilets is one of the major issues in post-disaster situations," said Itsuki Nakabayashi, head of the study panel.

The group is advising Tokyo residents always to carry pocket-sized tissue papers or plastic bags for emergency toilet use and urged companies to have portable toilets on standby.

The Japanese government has warned an earthquake with a magnitude of around seven could hit the capital in the coming decades.

Japan is hit by 20 percent of the world's powerful earthquakes.

Much of Tokyo was destroyed in the 1923 Great Kanto Earthquake, which left 142,807 people dead or unaccounted for.