Industrial firms were originally ordered to restrict output from Aug. 17 until Aug. 24, but formal curbs have been extended until Aug. 25, according to a notice issued by the Chongqing authorities on Wednesday.

Curbs will be gradually relaxed "in an orderly manner" once weather conditions have improved, it added.

Pangang Group Vanadium & Titanium Resources Co Ltd told the stock exchange in a filing on Wednesday its Chongqing subsidiary had received the notice and would continue to suspend production.

"The specific recovery time will be subject to the notification of relevant departments in Chongqing," it said.

Chongqing and Sichuan province, and large swaths of China, have been broiling under several days of 40 Celsius (104 Fahrenheit) heat - causing major drought in some areas and forest fires.

Although national forecasters reduced their heat alert level from "red" to "orange" from Tuesday, temperatures are still expected to exceed 40 Celsius in Chongqing, neighbouring Sichuan and other parts of the Yangtze delta until the weekend.

Power rationing in the region has shortened mall hours in Chongqing and impacted firms across sectors including battery makers and solar panel equipment manufacturers. Toyota Motor said it had used an in-house generator at its Sichuan plant to resume operations.

(Reporting by Beijing newsroom and David Stanway in Shanghai; Editing by Lincoln Feast.)