Pollution in Stour Brook kills thousands of fish
The "entire fish population" in a stretch of a Suffolk brook is likely to have been killed by pollution, the Environment Agency said.
About 2,000 large fish and thousands of smaller ones have died in Stour Brook near Haverhill since Tuesday.
Anglian Water said a problem with its Haverhill water recycling centre had led to sewage entering the brook.
The Environment Agency confirmed the fish died "from a lack of oxygen caused by a pollution".
A spokesman warned people to keep their dogs away from the water.
Anglian Water said it was doing everything it could to "remedy this situation".
"Our alarm systems alerted us to the problem immediately, and we dispatched engineers to the site to investigate the problem.
"We are working closely with the Environment Agency to assess the situation, and to protect and restore the brook."
Environment Agency staff have been aerating the brook, which leads to the River Stour, and oxygen levels have now returned to normal.
A spokesman confirmed pollution had killed the fish in a 3km (1.9 miles) stretch of the brook but it had not reached the river itself.