The “precautionary measure” comes after traces of cryptosporidium, a parasite which can cause gastrointestinal illness, were detected during routine tests facility in Preston.
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.
People can continue to use tap water without boiling for general domestic purposes such as bathing, flushing toilets or washing clothes.
UU chief scientific officer Martin Padley said: “The advice is being given purely as a precautionary measure as we carry out additional tests.
“We apologise for the inconvenience but the health of our customers is absolutely paramount.”
UU is monitoring water supplies and is hoping to be able to lift the boil water advice “as soon as possible”.
A version of this article first appeared on WWTonline.