More than 300,000 households in Lancashire have been warned they may have to boil drinking water until at least Wednesday.
“Low levels” of a microbial parasite remain in the supply, United Utilities said.
There was one clear test overnight, but further tests are required before the boil notice can be lifted, the firm said.
Samples are said to have “improved” but not returned to normal.
The alert affects Blackpool, Chorley, Fylde, Preston, South Ribble, Wyre and the Mellor area of Blackburn.
Investigations continue into how traces of cryptosporidium, which can cause stomach upsets, got into Franklaw water treatment works outside Preston
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