The government warned that a massive sudden blackout could occur in the Tokyo area Thursday evening and night as cold temperatures increased power demand, although a major blackout was likely to be averted.

The development came even though Tokyo Electric Power Co. implemented rolling blackouts for the fourth consecutive day Thursday, mainly in the Kanto region surrounding Tokyo, to cover substantial power shortages after last week's massive earthquake and tsunami crippled nuclear power plants.

As an emergency measure, the Land, Infrastructure, Transport and Tourism Ministry asked railways serving the metropolitan area to reduce the number of trains from evening to night, when the use of electricity was expected to peak for the day amid the commuter rush.

Railways, including East Japan Railway Co., Tokyo Metro Co., Odakyu Electric Railway Co. and Tokyu Corp., said they would cut the number of local trains in the Tokyo metropolitan area from 5 p.m. Thursday.

Among other businesses, many companies sent workers home early, while some Tokyo department stores closed earlier than scheduled. In addition, convenience store chain FamilyMart Co. planned to halve the fluorescent lighting at its stores.

Economy, Trade and Industry Minister Banri Kaieda said a massive blackout could occur despite the scheduled power rationing in Tepco's service area as demand had already neared the supply capacity for Thursday during the morning hours.

Tepco said power demand temporarily hit 33.3 million kw, against its supply capacity of 33.5 million kw for Thursday, between 9 and 10 a.m. It said demand was around 29.0 million kw as of 5 p.m. and demand stood below supply during the anticipated peak time between 6 and 7 p.m.

The utility said some areas may experience sudden blackouts if excess demand over supply continued, but explained that a slight exceeding of supply would not immediately lead to a widespread metropolitan blackout.