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A person walks through a cooling mist spot during a heat wave in Tokyo, on June 28.Eugene Hoshiko/The Associated Press

Japan braced on Wednesday for its hottest day yet of a record-breaking heat wave, as Prime Minister Fumio Kishida called for a ramp-up of nuclear power use amid fears of a shortage of electricity to keep air conditioners whirring.

As some manufacturers announced plans to scale back production to save power, temperatures of around 40 C were predicted in areas surrounding Tokyo on the fifth day of the capital’s worst June heat wave since records began in 1875.

The Japan Meteorological Agency forecast that temperatures there won’t drop back to 30 C until July 5.

“The electricity demand and supply situation is expected to be the toughest in the last three days (of this week),” an industry ministry official told reporters.

All additional measures to boost supply had been incorporated, according to national grid monitor OCCTO, whose mid-morning estimate showed the reserve ratio of power generation capacity for the Tokyo area likely to fall as low as 2.6 per cent between 4.30 p.m. and 5 p.m. on Wednesday – under the minimum 3 per cent threshold deemed necessary to ensure stable supply.

The government signalled a power shortage warning for the fourth consecutive day on Thursday for Tokyo and surrounding areas although the supply-demand balance would likely be less tight with additional capacity coming on stream.

Prime Minister Kishida said he would do his utmost to secure enough supply, telling a news conference he would make the greatest possible use of nuclear power as long as safety was assured.

Most of Japan’s nuclear plants have been halted since the March 2011 tsunami that set off the Fukushima nuclear accident.

Meanwhile, power companies are rushing to restart thermal power plants that have been shut down and calls are rising for additional use of alternate energy sources, including restarting reactors.

As officials again called on households to save electricity where possible – without stinting on air conditioning where it would endanger the health of the vulnerable – auto parts maker Yorozu Corp said it would close its manufacturing plants for at least two days a month from July through September.

Seven & i Holdings said it was asking 7-Eleven stores in and around Tokyo to take further power-saving measures such as dimming signboards and turning off ventilation fans when deep-frying equipment was not in use during the 3-8 p.m. crunch time.

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