A Japanese train driver faces possible punishment after he left the cockpit of a speeding bullet train for several minutes to go to the toilet.

He had asked a conductor, who did not have a driver's licence, to man the train which was travelling at 150km/h (93mph), according to local media.

The Hikari 633 was carrying 160 passengers at the time. The incident reportedly did not affect the journey.

But the railway company has reported it to authorities and apologised.

The Central Japan Railway Company (JR Central) said the incident took place on Sunday morning while the train was travelling in the central Shizuoka prefecture.

The 36-year-old driver, who has not been named, had suffered a stomach ache and needed to use the toilet urgently.

He called a conductor into the cockpit to man the controls, and then left for about three minutes to use the lavatory in a passenger cabin, JR Central said.

The company's rules state that if drivers feel unwell they must contact their transport command centre. They are also allowed to ask a conductor to take over the controls, but only if the conductor has a driver's licence.

The driver and the conductor now face possible disciplinary action, said JR Central.

Senior official Masahiro Hayatsu told reporters: "It was an extremely inappropriate act. We apologise."

Japan's famously efficient railways are strictly regulated with high safety standards, and rail accidents are rare.

The last major incident happened in 2005 when a train derailed in the western city of Amagasaki, killing 107 people.

The Shinkansen, which is Japan's bullet train rail network, has never had an accident in its 57-year history.