A clean‑up operation is under way after about 1,750 gallons (8,000 litres) of milk spilled into a stream.
The Environment Agency said milk was accidentally released from a farm into a watercourse that feeds into the River Exe, near Tiverton in Devon, at about 03:00 GMT on Tuesday.
Officers said the farmer and a delivery driver raised the alarm immediately, helping them “put in place a plan to reduce the risk of environmental harm”.
The agency said milk could strip oxygen from water, which could lead to fish dying in large numbers, but it added it had found environmental damage “should be limited”.