SHANGHAI (REUTERS) - For the third time this summer, Shanghai issued its highest alert for extreme heat as temperatures appeared set to test records in China's most populous city on Thursday (July 14).

The city of 25 million residents announced a red alert, indicating temperatures were expected to rise to at least 40 deg C over the next 24 hours.

The city has issued three red alerts in the past five days, though it is a relatively rare occurrence as Shanghai has issued 17 such alerts since record-keeping began in 1873.

On Wednesday, the city sweltered as temperatures rose as high as 40.9 deg C, matching a record set in late July, 2017.

Half of China has been affected by the unusually hot spell during the past month. The Yangtze River basin - encompassing megacities from Shanghai to Chongqing in the heartland - has suffered heat waves over the past week.

With higher demand for air-conditioning, the load on the power grids of seven provinces and regions has reached a record high, according to state media.

China's maximum power load hit an all-time high of 1.22 billion kilowatts on Tuesday, the National Development and Reform Commission (NDRC) said on Thursday.

The state planner said it was making every effort to ensure energy supply during the summer peak demand period.