Around 6.8 million tonnes of the valuable minerals, used in electric cars, iPods and lasers, are sitting under the seabed near a far eastern Japanese island, Tokyo University professor Yasuhiro Kato told AFP.

He said mud samples taken from an area near Minamitorishima island, some 2,000 kilometres (1,250 miles) southeast of Tokyo, indicated deposits amounted to around 220 times the average annual amount used by industry in Japan.

The seabed contained a substantial amount of dysprosium — a rare earth mineral used in the engines for hybrid cars, he said.

“Specifically on dysprosium, I estimate at least 400 years worth of Japan’s current consumption is in the deposits,” said the professor, who examined mud samples taken from the seabed around 5,600 metres (18,300 feet) down.

“We can start drilling in the mud, using oil extraction technology, within three years at the earliest and start producing rare earth minerals within five years,” he said.

The find would be the first time large scale rare-earth deposits had been discovered inside Japan’s exclusive economic zone, local media said.

Rare earths are used to make a wide range of high tech products, including powerful magnets, batteries, LED lights, electric cars, iPods, lasers, wind turbines and missiles.

China currently produces more than 90% of the world’s supply of rare earths, but has clamped down on exports of them in a move Beijing says is aimed at protecting its environment and conserving supplies.

But Japan, the European Union and the United States claim China is unfairly benefiting its own industries by restricting exports.

The confirmation of a significant find of rare earths in Japanese territory would be welcome news for Japan’s hi-tech industries who were caught in a political spat between Tokyo and Beijing in 2010 when China squeezed supplies.

“I would like to see the Japanese government recognise the existence of the rare earth deposits and soon start making investment in developing the area,” said professor Kato.