The Environment Agency received a call from a member of the public on Monday after he reported seeing dead fish in the Wandle between Beddington and Merton.
Officers who visited the site found dead fish including barbel, chub and eels and it is expected that other species have been affected by the spillage.
The site was monitored closely overnight and Environment Agency teams are onsite today collecting further evidence. A clean up operation is being jointly organised with Thames Water Utilities Limited.
Mike Denbigh, Environment Management team leader, said:
“This is a serious incident. We will not be able to confirm the exact nature of the pollutant that has entered the stream until water samples are analysed. The early notification from the member of the public enabled us to respond quickly. As a result we feel confident that we will discover the cause of this incident."
Members of the public can report pollution incidents by calling the Environment Agency’s emergency hotline on 0800 807060.
Pollution causes fish kill on London river
UK - Officers from the Environment Agency are investigating reports of a pollution incident that has killed at least 2,000 fish along a 5km stretch of the River Wandle in south London.