Dead fish have been found on a river in Norfolk where a large stretch of white foam appeared, the Environment Agency has confirmed.

Images shared by the agency on Saturday showed the foam covering an area of the River Thet.

The Environment Agency said its officers had “seen small dead fish and fry”. It said it was investigating the incident and “the pollutant is an unknown substance”.

Specialist teams were sent to collect samples from the river, which flows through Thetford, on Saturday with the aim of finding the source of the pollution and preventing further discharge.

The source of the foam, which remains visible, was detected and further discharge stopped.

Firefighters also attended and Norfolk fire service warned people not to enter the water or allow their pets to swim in or drink from the river.

Anglian Water confirmed there was a “third party” involved and was not from any of their “equipment or assets in the area”.

An Environment Agency spokesperson said that further foam might occur but had said it would have “little environmental impact as it dissipates”.

They said: “Environment Agency officers are continuing to investigate the cause of a large volume of foam found in the River Thet in the centre of Thetford yesterday.

“Our specialist teams have collected samples, detected the possible source, and prevented any further discharge. There will be an ongoing investigation to try to ensure that this does not happen again.

“It is possible that we might see a second foam plume today and the foam will continue to travel down the river. It is still visible but will have little environmental impact as it dissipates.”

On Sunday the agency said it would wait until the pollutant had gone to assess the longer-term effects.

They added that its officers had attended an incident involving foam on the River Thet on 23 December 2024 but were unable to identify the source on that occasion.

In 2023 an investigation by the Guardian and Watershed found that toxic substances called Pfas, which include firefighting foams, had been found polluting the protected River Wyre in Lancashire at “extremely high levels”.