An investigation is under way over whether sewage was illegally released into Norwich's River Wensum.
Environment Agency officers confirmed they are investigating Anglian Water over potential breaches of permits involving storm overflows.
Rachel Storr, from the Environment Agency, said, while working through 2020 data, the agency had identified four spills into the River Wensum, which might have been illegal.
She told a Norfolk County Council meeting, where storm overflows were being discussed: "We did identify potential non-compliances with the four on the Wensum that we looked at and that is still under investigation."
She said Anglian Water will be asked to provide information as part of the investigation, but the Environment Agency could take enforcement action if appropriate.
The backlog for the following years is still being analysed, which could spot further breaches.
Permits allow water companies to use storm overflows in certain conditions when there is too much water in pipes, such as after heavy rain, to stop it backing up into people's homes. But it is illegal to use them at other times.
Safety release valves open to reduce the pressure, releasing excess, untreated, water directly into rivers or the sea.
READ MORE: Sea Scouts fear for future over River Wensum pollution
Green county councillor Jamie Osborn said: "In the Wensum, in a short two-mile stretch, in 2019/20, there were more than 1,000 spills, lasting more than 16,000 hours.
"I don't think it's acceptable that this is happening and it's taking a long time to get any enforcement action."
Ms Storr said the majority of storm overflow use was permitted and legal.
READ MORE: Norfolk Rivers Trust volunteers analyse Wensum water quality
Dr Robin Price, from Anglian Water, said the company was aware of data from the River Wensum, which suggested a high number of spills.
But he said the company had verified it and it appeared to be a "data issue", rather than "genuine spills".
He said Anglian Water would "cooperate fully" with the Environment Agency over the investigation.
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