In June the local environmental protection bureau detected dangerous levels of arsenic in the lake. A nearby firm had not built proper containment pools for its waste, and years of accumulated arsenic were leaching through into the groundwater and the lake, an investigation found.

The factory has been closed and its managers detained.

Odourless, tasteless, colourless and easily soluble, arsenic has long been a feared poison. A heavy dose can kill, while small amounts may also sicken people gradually.

The water would not be safe for at least three years, the official Xinhua agency said.

China has been struggling to clean up its battered environment after years of promoting growth at almost any cost, but corruption and poverty mean it still faces regular problems.

Earlier this month over 250 people were killed when a waste containment pond at a mine collapsed, creating a roaring tide of mud that engulfed homes and a market.

Many of China's major lakes are also choked with pollution or over-exploited, including Dianchi which is also in Yunnan and has been dubbed the "mother lake" of the nearby provincial capital.

The government has pledged to return them to their original state but only set a target date of 2030.

(Reporting by Emma Graham-Harrison; Editing by Jeremy Laurence)