Cold temperatures have been blamed for the surge in power demand in and around metropolitan Tokyo, which is already under area-by-area rationing, Kyodo News reported. About 10 million households could be affected by any total blackout.
Economy, Trade and Industry Minister Banri Kaieda said the blackout could occur as demand in the area served by Tokyo Electric Power had already neared available supply as of Thursday morning.
The government urged more conservation efforts by businesses and individuals.
Some of the major train operators planned to reduce the service in the evening.
The power rationing was expected to continue until next month.
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