United Utilities is advising customers in Blackpool, Chorley, Fylde, Preston, South Ribble and Wyre to to boil their water for all drinking, food preparation and brushing teeth as a precaution until further notice.
The company say people can continue to use tap water without boiling for general purposes such as bathing, flushing toilets or washing clothes.
Routine tests at Franklaw water treatment works detected traces of cryptosporidium which can cause gastrointestinal illness.
Martin Padley, Chief Scientific Officer at United Utilities, said: “The advice is being given purely as a precautionary measure as we carry out additional tests today.
To find out if your postcode is affected, click here