More than 300,000 households in Lancashire will have to boil drinking water into next week, United Utilities has said.
Routine tests by the company found "low" traces of the microbial parasite cryptosporidium at Franklaw water treatment works outside Preston.
The alert affects Blackpool, Chorley, Fylde, Preston, South Ribble and Wyre.
Cryptosporidium can cause gastrointestinal complaints such as diarrhoea and abdominal cramps.
People with weak immune systems are likely to be more seriously affected.
United Utilities said tests showed that traces of the parasite are reducing.
Some customers have criticised United Utilities for not informing customers quickly enough.
Blackpool resident Robert Parker said local shops had sold out of bottled water.