Sutton and East Surrey Water will be issuing a boil notice to households and businesses within its Sutton supply area. Around 50,000 properties and approximately 125,000 people are affected

The decision to issue the boil notice is precautionary and follows close consultation with the Health Protection Agency and other public health advisers.

Anyone drawing water for consumption or the preparation of food in the affected area supplied by Sutton and East Surrey Water bounded by Morden, Worcester Park, Carshalton, Wallington, Ewell, Stoneleigh, Sutton and Cheam should boil the water before use. This follows an incident at the company's Cheam treatment works on Friday when the exceptional level of torrential rain that fell caused flooding at the works and a small quantity of rain water entered a tank used to treat water. Unfortunately this water went into supply before it was detected, when the works was immediately shut down.

The water tank involved holds heavily chlorinated water as part of the final treatment process. As such it is designed to automatically eliminate most organisms which may affect water quality. The rain water would also have been heavily diluted.

Mike Hegarty, Sutton and East Surrey Water's Operations Director, said: We have reason to believe that some of the water which went into supply may not have met our quality standards and, as a precaution, we are asking everyone in the affected area to boil their water before consuming it until further notice.

The water is perfectly safe to use for other purposes including washing, bathing and toilet flushing.

We believe that all the water which may have contained some of the rain water has already passed through our supply system, but we are following the precautionary principal. Some of the water may still be in people's supply pipes or, if they have one, their water tank in the roof.

Boil notices are being printed and will be delivered, as soon as possible, to every property within the affected area. Sutton and East Surrey Water is also sending vans with loud hailers into the area to warn customers to boil their water.

All this takes time, said Mike Hegarty. We are hoping that our customers will hear about the notice through the media. We have alerted hospitals in the area, GPs and other health officials and are endeavouring to contact everyone in the area who is registered with us who may be vulnerable.

Public health consultant of the south west London Health Protection Unit, Doctor Paul Crook, said: The Health Protection Agency has advised that people need take no further action apart from ensuring their water is boiled before drinking, as described above. The HPA considers the risk to people's health to be low. If anyone were to be taken ill they should seek medical advice as normal, via NHS Direct or their local doctor.

If customers have any health concerns regarding this advice or require any further medical advice then they should call NHS Direct on 0845 4647