An EA spokesman said: “Pollution incidents are unacceptable and we share the public anger.
“Specialist officers are investigating… They are working with Thames Water to identify the source, after which, next steps will be considered.”
A Thames Water spokesperson said: “Initial investigations suggest that an oil-based substance may have entered the river via a roadside gully, likely by a third party.
“Our teams are continuing to investigate the source and extent of the incident, and we are working closely with the Environment Agency to assess the environmental impact.”