A serious pollution incident has killed thousands of fish in the River Pool in Bromley and Lewisham.
The Environment agency said it was a Category 1 incident that had killed European eels, a critically endangered species, sticklebacks, gudgeon and stone loach.
The EA said some large fish, such as chub, roach and dace were also killed.
The pollution has affected a large stretch of the Pool from Cator Park down to the confluence with the River Ravensbourne in Catford
The cause of the pollution incident, first reported at the end of June, is not yet clear.
Public drinking water supplies have not been affected, according to the Local Democracy Reporting Service on the BBC news website. But it said the EA had advised members of the public and their animals to avoid the affected stretch of the river.
The agency’s environment officers are monitoring water quality and its fisheries team is assessing the extent of the impact on river wildlife
Thames Water, which operates the local drainage network, said it was investigating the pollution incident.
The company has deployed aeration at key locations along the River Pool to increase dissolved oxygen levels and help mitigate further impacts.
A Thames Water spokesperson said tests were currently being carried out and it “understands the concerns residents will have”.
The EA said its monitoring showed water quality was now slowly improving. It said it would continue to monitor water quality and would work with partners to support the recovery of the river
The European eel is a critically endangered, migratory fish known for its astonishing life cycle. Spawning in the Sargasso Sea, these eels drift thousands of miles to Europe’s rivers, where they mature for decades before returning across the Atlantic to spawn and die.
The River Ravensbourne in Ladywell Fields is not thought to have been affected although members of the park user group LFUG expressed concerns. “Pollution has a habit of drifting downstream.”