Millions of people in Tokyo are being urged to turn their lights off to save electricity as an “unusually intense” heatwave sees power supplies dwindle.

Japanese officials say demand for electricity is surging after temperatures rose to a record-breaking 40.2C (104.4F) on Saturday – the highest ever for June – in Isesaki, located 53 miles northwest of the capital.

People have been asked to use air conditioning “appropriately” as more than 250 in Tokyo were reportedly taken to hospital with heatstroke over the weekend.

A 94-year-old man died of suspected heatstroke after he was found in a non-air conditioned room in Kawagoe city, around 12 miles to the northwest.

A heatstroke alert was issued in six of the country’s 47 prefectures by the Japanese Meteorological Agency and environment ministry on Saturday, recommending people remain indoors to stay cool.

The ministry of economy, trade, and industry (METI) has called on people living in and around the capital to conserve power from 3-6pm, in particular between 4-5pm.

“Please save as much power as possible, such as by turning off lights that are not in use,” METI said in a statement.

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METI director of electricity supply policy, Kaname Ogawa, added: “We are struck by unusual heat for the season.

“Please cooperate and save as much power as possible.”

More than 37 million people live in the metropolitan area of Tokyo, thought to be the most populous in the world.

The crisis has intensified as a result of the earliest end to the annual summer rainy season since the meteorological agency began keeping records in 1951.

The rains usually alleviate the summer heat into July.

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It comes as weather forecasters warned this summer will be hotter than expected in northern, eastern and western Japan, due to factors including global warming.

METI estimates that excess generating capacity will plunge to 3.7% on Monday afternoon in Tokyo and eight nearby prefectures in eastern Japan.

A buffer of 3% is considered the minimum requirement for a stable power supply.

The temperature in Isesaki on Saturday broke the previous record of 39.8C on Japan’s hottest day, 24 June 2011.

Another weather station in central Tokyo logged 35.4C earlier in the day, marking the earliest arrival since records began in 1875 of an above-35C day in the capital – considered extreme heat in Japan.

The country also faces a reduced energy supply following the 2011 meltdowns in Fukushima and the closure of old coal plants in a bid to reduce carbon emissions.

Japan also faces a potential fossil fuel shortage due to sanctions against Russia in the wake of its invasion of Ukraine.

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