Another 500,000 tonnes of sewage has poured into the River Thames following a night of heavy rain.
Thousands of fish died after storms forced sewage into the Thames
It overflowed at Abbey Mills pumping station into the River Lea, south of Stratford, in east London, and then into the Thames.
It was the latest incident in the last 16 days, during which time almost five million tonnes of untreated sewage has polluted the river.
The Environment Agency has warned that it is likely be an ongoing problem.
The recent overflows date back to 3 August when 600,000 tonnes of raw sewage flooded into the river, suffocating hundreds of thousands of fish.
The river from Kew to Woolwich has been affected by the discharges, which drastically reduce vital oxygen levels that support fish life.
The latest discharge prompted the Environment Agency to instruct Thames Water to re-launch the oxygenating vessel in a bid to limit the fallout.
Richard Oatley, environment officer at the Agency said: "The oxygenating vessel is working hard to relieve the situation and we do not expect there to be a significant fish kill.
"However, Londoners can only expect the problem of sewage in the River Thames to be an ongoing health, amenity and environmental problem."