The notice is a precautionary measure that was issued on Friday after traces of cryptosporidium, a parasite which can cause gastrointestinal illness, were detected during routine tests facility in Preston.
UU said it is “making good progress and the situation is improving” but that the boiled water notice will remain in place.
The company is flushing out the water network surrounding the Franklaw treatment works to remove and traces of the parasite and an investigation has been launched to find out how the water was contaminated.
A company spokesperson told Utility Week: “We have to be absolutely certain that are no traces in the system of cryptosporidium before we remove the boiled water notice.
“The decision will be taken of when to lift the notice in consultation with Public Health England and we want to lift the notice as soon as we possibly can but we can’t take any chances with public safety.”
In a statement the company added: “This remains a very unusual incident; the risk to health is very low but we will not take chances with public health so please do continue to boil your water as a sensible precaution.”
UU said the traces were low but it was advising customers in Blackpool, Chorley, Fylde, Preston, South Ribble and Wyre to boil their water as a precaution.