Chinese Premier Wen Jiabao has demanded a thorough investigation of the Taihu Lake crisis, which has affected the drinking water supply of about 2 million residents in east China.

"The pollution of Taihu Lake has sounded the alarm for us," Wen said in a directive to a symposium held by the State Council on Monday.

Taihu Lake, which was once a scenic spot famous for its aquatic life, including shrimp, lily and water chestnuts, has been heavily polluted by industry, agriculture and domestic waste.

Wen said efforts had been made to reduce pollution in the Taihu Lake in recent years. "But the problem has never been tackled at the root."

He asked participants of the symposium, including officials from central and local governments, environmental workers, scholars and researchers, to thoroughly investigate the Taihu Lake crisis and come up with concrete measures to protect the environment.

Vice Premier Zeng Peiyan, who attended the symposium, demanded governments at all levels make every effort to ensure the safety of drinking water.

Zeng asked local governments to continue collecting the blue-algae that has discolored the lake and to divert more water from the Yangtze River to flush out the pollution.

He also asked environmental watchdogs to strengthen supervision and punish factories that discharge pollutants into Taihu Lake.

The Taihu Lake crisis started last month, as the low water level and the accumulation of waste and untreated sewage triggered the rapid growth of blue algae, turning the water putrid and cutting freshwater supplies to more than 2 million residents.

Authorities diverted water from the Yangtze River to dilute the lake water and used chemicals to treat the algae.

Workers have collected 6,000 tons of blue-algae from the lake, and local health authorities say that tap water once again meets drinking standards.

"The environment of Taihu Lake impacts on the economic and social development of the Taihu valley and the Yangtze Delta," Vice Premier Zeng said.

Zeng asked local governments to better balance economic development and environmental protection and to make environmental protection a higher priority.

Local governments are also required to improve urban sewage treatment and upgrade sewage treatment facilities in the countryside to ensure that rural waste will not flow into the lake without being treated.

Zeng also demanded a time limit for environmental watchdogs to close heavily polluting factories surrounding Taihu Lake and asked for a higher environmental standard for new projects.

Earlier reports said that authorities have ordered all towns around Taihu to establish sewage treatment plants and insisted that chemical factories meet a new water emission standard by the end of June 2008.

Chemical factories that fail to meet the new water emission standard will have their production suspended. They will be shut down permanently if they still fail to meet standards by the end of next June.

Editor: Yan