Shanghai firms are being made to switch operations from day to night

The authorities made the order as the continuing heat wave and subsequent high usage of air-conditioning put pressure on electricity supplies.

About 2,000 Shanghai businesses are now having to work at night as daytime temperatures remain above 38C.

The city's booming economy grew 14.8% in the first half of 2004.

Scorching demand

A further 3,000 companies are being asked to introduce some night-time operations, but there have been no power cuts so far, said Chinese state television.

It warned that the heat wave was set to continue.

"Owing to predicted high temperatures next week, Shanghai will from July 19 begin a new round of electricity rationing," it said.

With electricity usage in Shanghai surging to record levels, hotels, shopping malls and other public places are also being told to limit the use of their air-conditioning.

Economists have warned that as China's economy continues to grow at breakneck speed, it is putting ever more pressure on power supply.