A Morden brook suffered a pollution spill at the weekend.
A witness said a thick oily substance appeared to have entered the Pyl Brook in Morden at the weekend.
The Environment Agency said its officers were at the scene today assessing the impact of the spill.
Watch a video of the spill
Last summer hundreds of fish were killed after a chemical leak from a sewage treatment work at the River Wandle.
Thames Water, which was responsible for September's spillage, said it would be working to try and bring the river, a popular haunt for anglers, back to full health.
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John Sutton, fisheries and biodiversity team leader for the Environment Agency, said the last pollution spill was likely to continue to have an effect on the Wandle's habitat for many years.
Did you see the spill? Call the newsdesk on 020 8330 9555.
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