Japan sizzled through its hottest day on record, as a heat wave claimed at least six lives across the country and spurred fears of an electricity shortage.

The mercury hit 40.9 degrees Celsius (105.62 degrees Fahrenheit) in the western city of Tajimi, breaking a previous national record of 40.8 degrees Celsius set in 1933, according to the Meteorological Agency.

Temperatures also soared to new records in Tokyo and across the country, spurring holidaymakers to take cover indoors.

Six people died overnight in eastern Japan from heat-stroke, including a 13-year-old boy who collapsed in Tokyo after playing basketball, the Yomiuri newspaper reported. Many others were admitted to hospital.

Tokyo Electrical Power Co. warned of a power shortage as people turned up their air conditioners in the heat.

The company has been firing up old thermal power stations and buying electricity from rivals after a strong earthquake ravaged its largest nuclear power reactor, reducing its electricity output by more than 10 per cent.