About 19,000 litres of milk were spilled when the tanker hit a bridge over a stream flowing into Rudyard Lake near Leek.

Firefighters spent the day pumping milk out into nearby fields and building dams to prevent it from flowing into the lake.

David Thomas of the Environment Agency said: "Milk is a highly polluting substance."

Beauty spot

The brook at Rushton Spencer, where the spillage occurred, has been described by one eye-witness as being "white with milk".

Some 50,000 fish are in danger if the milk enters the reservoir, which is a popular beauty spot and recreational boating lake.

Mr Thomas explained why the spillage is so dangerous.

"Milk is a highly polluting substance in terms of its oxygen demand.

"In real terms that means the bacteria will feed on it and use up oxygen that would otherwise be used by fish and other animals living within the watercourse.

"This is a road traffic accident in a remote location right on top of a watercourse - it's a worst-case scenario."

The driver of the tanker was taken to hospital with minor injuries after the crash on Wednesday morning.