Hundreds of thousands of people in Lancashire have been warned they should continue boiling their tap water before use for several days.
Tests at a water treatment plant found traces of the microscopic bug cryptosporidium, which can cause sickness and diarrhoea.
Water firm United Utilities, which first issued the boiling water advice on Thursday, has warned customers that the situation will continue for a number of days.
The company wrote on Twitter: "It's going to (continue) into next week, carry on boiling, we will be contacting customers about compensation."
The affected households cover a large area of Lancashire including Blackpool, Preston, Chorley and the Fylde coast.
United Utilities is advising customers to boil their water and then let it cool for all drinking, food preparation and teeth brushing as a precaution until further notice.
Residents can continue to use tap water without boiling for general domestic purposes such as bathing, flushing toilets or washing clothes.
The warning comes after routine tests at Franklaw water treatment works in Preston detected traces of cryptosporidium and although the traces are low the company is advising customers to boil their water.
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.
"We apologise for the inconvenience but the health of our customers is absolutely paramount."
United Utilities say it is monitoring water supplies and is hoping to be able to lift the boil water advice as soon as possible.