The Environment Agency has been combating a “pollution incident” in a river which flows through a university city.
Officials said the pollution was reported at Baits Bite Lock on the River Cam in Cambridge on Friday.
A spokesman said “oil-absorbent booms” had been positioned in the river to treat the incident.
He added: “Anyone with information about a pollution incident can call [the agency].”
“We received reports of a pollution incident at Baits Bite Lock on Friday afternoon and officers attended the scene,” said the Environment Agency spokesman.
“We have deployed oil-absorbent booms and will continue to monitor the site.”
In March 2019 researchers tested the water in the River Cam a bid to identify the source of bacterial infections affecting rowers and swimmers.
A month before, the Cambridge Fish Preservation and Angling Society (CFPAS) won a claim against Anglian Water over fish deaths after a discharge from a treatment works.
CFPAS alleged, in a civil claim at Peterborough County Court, that pollution emitted from a recycling centre had caused fish stocks in the River Cam to decline.
Anglian Water admitted it was to blame for the fish deaths and agreed to pay CFPAS undisclosed damages.
CFPAS holds fishing rights downstream of the centre and alleged numbers of big bream at Baits Bite Lock had dwindled.
Anglian Water said it had acted within its legal permit.