Managing director Peter Darby told BBC News Online that water should be boiled as a precaution while investigations are carried out.

The cause for concern is that something quite exceptional got into the source

Peter Darby, Folkestone and Dover Water

The "boil order" will only be lifted 24 hours after the water is given the all clear.

"The concern is that this looks likely to be linked to the heavy rainfall over the last few days, and more heavy rain is forecast for the next two days," said Mr Darby.

The "contaminated" source has been shut down and water is being redirected from other sources.

Mr Darby said the treatment works had been working properly.

"The cause for concern is that something quite exceptional got into the source," he said.

"We have to be confident that the water in the system is safe to drink."

He said people drinking water from the tap without boiling it first could risk stomach upsets.

There are no water shortages at present, but Folkestone and Dover Water - along with Three Valleys Water which serves an area North of London - is urging customers to use water sparingly.