The Environment Agency have said they are looking at possible enforcement action after the River Nene in Peterborough was polluted, killing hundreds of fish.
Residents living at Riverside Mead in Stanground spotted the water was a cloudy, milky colour on Monday - and on Tuesday found dozens of dead fish floating in the water.
A number of fish also washed up in residents’ gardens.
It is believed around 400 fish were killed.
Today Environment Agency spokesperson said: “Our officers have responded to reports of dead fish at Stanground Lode, off the River Nene, and our investigations have found elevated levels of ammonia and low oxygen in the water.
“We have identified the source and cause of the pollution, which has been stopped and isn’t posing any further risk to the environment. Due to ongoing investigations and possible enforcement action, we’re unable to give further details at this time.
“As always, we’d remind people to please report fish in distress or suspected pollutions to us as soon as possible so we can investigate – call our 24-hour incident hotline on 0800 80 70 60.”
The spokesman added: “We believe this is an isolated incident which is not posing further risk to the environment, nor a risk to drinking water. The level of ammonia and oxygen levels in the water are now stabilising.
“By the time we received the reports, the fish were already dead so there was nothing we could do. Anyone who suspects fish in distress, fish deaths or pollution incidents should report them as soon as possible, so we can investigate and have as much opportunity as possible to take the appropriate actions.
“We would normally advise people not to let their pets drink from open water, as it could contain substances – even natural ones like algae – that could make them ill. This advice still stands as we are investigating this incident.”