Initial investigations by the Environment Agency and Thames Water indicate that the likely cause is the prolonged period of exceptionally hot weather, which has resulted in unusually high temperatures significantly depleting oxygen levels in the lake.
Similar incidents have been seen across the country during the recent heatwave.
Merton Council says its officers are working closely with partners to minimise any further impact. This includes installing aeration equipment to help increase oxygen levels in the water.
The council says there is no evidence to suggest that pollution is responsible, although investigations and water quality monitoring are continuing.
Dead fish are being removed from the lake as quickly as possible to help protect water quality and minimise any impact on wildlife.
As a precaution, people are being asked not to enter the water and to keep dogs and other pets out of the lake until Monday while conditions stabilise and clean-up work continues.
