Polluted run-off from a landfill has contaminated water supplies in Selangor, prompting widespread water cut-offs.

Some two million consumers were left without water on Saturday for more than 12 hours as authorities scrambled to remedy the situation.

Rozali Ismail, the executive chair of Syabas, a private water treatment and supply company for the state, said levels of ammonia in the river providing water to the areas were "very dangerous."

He said the run-off occurred after heavy rains and the collapse of an earthen wall separating the dump site from a river tributary.

"Someone must be responsible and make sure the rivers are clean," he was quoted as saying in the New Straits Times .

The contamination was the second case in western Selangor in recent months.

In February, residents in Kuala Lumpur and surrounding areas were treated to foul smelling tap water after a river supply was polluted by leakage from a landfill and pollution from activities along the river's banks.