Hundreds of thousands of people in Lancashire are being warned to boil their tap water before use to kill a microscopic bug.
Tests at a water treatment plant have found traces of cryptosporidium, which can cause sickness and diarrhoea.
The warning covers a large area of Lancashire including Blackpool, Preston, Chorley and the Fylde coast.
Water firm United Utilities are advising all customers to boil their water 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.