Traces of a diarrhoea-causing bug have been found in tap water in the Northampton and Daventry areas.
Anglian Water has this morning issued an urgent warning to people living in parts of the south of the county, urging them to boil water before drinking it.
About 108,000 homes – or 258,000 people – have been affected. The measure has been taken while scientists investigate routine samples that have shown evidence of cryptosporidium in the water supply.
A spokesman for the company said: "Anglian Water customers in the Northampton, Daventry and west of Daventry areas are advised to boil drinking water before use.
"Until further notice, Anglian Water customers in Northampton, Daventry and west of Daventry areas are being instructed to boil their drinking water before using it.
"Only once drinking water has been boiled and allowed to cool should it be used for drinking, preparing food or cleaning teeth. This advice also applies to pets.
"Tap water remains quite safe for washing, bathing and toilet flushing."
Alternative water supplies are currently being looked at for the affected areas, but due to the size of the area bottled water will only be delivered to special needs customers on Anglian Water's 'WaterCare' register as soon as possible.
A spokesman for the firm added: "Customers in the affected areas will receive further advice and should listen to regular updates which will be given through local media. Further updates will be available on the Anglian Water website, www.anglianwater.co.uk.
"The company apologises for the inconvenience this will cause, but stresses that the health and safety of its customers must come first."
Anglian Water is working with local authorities and Health Protection Unit to provide advice. The company has urged customers with worries about their health to contact their GP or call NHS Direct on 0845 4647.