The emergency was declared when an organic solvent found its way into the supply at the main treatment works in Worcester. People living in more than two dozen surrounding villages and parts of the nearby towns of Pershore and Malvern were also affected.
The alarm was raised after the water supply turned a dirty colour and gave off an unpleasant smell. Mothers of young babies were warned to throw away any supplies of bottle-feed which they had made earlier.
Seven Trent water officials toured schools and old people's homes delivering emergency supplies, and 250-gallon tankers were taken to the city's four hospitals.
Mike Sweeney, Seven Trent's area manager, said: 'No one should drink the water even if it has been boiled. We are carrying out tests to find out what has happened. It might be a simple answer or it might be more complicated.'
Police appealed to the public not to telephone them after their switchboards became jammed and they were unable to deal with normal emergencies.
The water authority set up emergency helplines to cope with demands for information.
Traffic jams formed just outside the city as householders with water carriers drove to the Malvern Hills 10 miles away to try to draw water from a natural spring.
The Worcester medical officer, Dr Bryan McLoskey, said: 'If anyone feels ill after drinking the water, they should contact their doctor immediately.'
Severn Trent Water said last night that the pollution was spreading over a 50-mile stretch of the river Severn from Bewdley in the north to Gloucester.
Officials warned that if the treatment works at Strensham had to be closed, there could be a water shortage throughout Hereford and Worcester.
'We have eliminated some things for which we were looking but we have not yet managed to identify the materials involved,' Mr Sweeney said.
'The pollution is now very, very extensive. The National Rivers Authority is trying to discover what it is and where it has come from.'Reuse content