Anglian Water has paid a £150,000 fine after sewage was released into the River Til in north Bedfordshire.
It happened in 2018 after contractors unblocking a sewer in Yielden caused the small rural Water Recycling Centre to become overwhelmed.
It resulted in sewage discharging into the river.
The impact to the River Til was limited to 660 metres, but ammonia levels were high.
Dissolved oxygen was also depressed at a level likely to impact fish but there was no evidence of any dead fish or invertebrates.
The Environment Agency said: “Anglian Water reported the pollution to the Environment Agency and took action to clean it up. Other measures were taken to prevent a similar incident happening in the future.”
The Environment Agency imposed a Variable Monetary Penalty (VMP) on Anglian Water Services Ltd of £150,000 and costs to the Environment Agency of £4,428.33. This was the first time a water company in England has paid a VMP in relation to a pollution incident.
Jeremy Hay, Senior Environment Officer at the Environment Agency, said: “Polluters should always be held to account, and, as much as our resources allow, we will always investigate significant pollution incidents and bring those responsible to justice.
“We are pleased with the success of this type of civil sanction, which we hope will change behaviour. It’s vital that future civil sanctions are not only proportionate but also an effective deterrent to all who pollute our rivers and waters.
“If members of the public are concerned about pollution, they should call our 24/7 incident hotline on 0800 80 70 60.”
