Investigations are under way into how water became contaminated, after a parasite was found in the supply in parts of Lancashire.
A health warning is to remain in place over the weekend and into next week, United Utilities have confirmed.
People in Blackpool, Chorley, Fylde, Preston, South Ribble and Wyre were being advised to continue to boil their water on Friday as a precaution, for all drinking, food preparation and brushing of teeth.
United Utilities is advising customers that the boiled water notice will remain in place throughout the weekend, and into early next week at least.
They say people can continue to use tap water without boiling for general domestic purposes such as bathing, flushing toilets or washing clothes.
The advice comes after routine tests at Franklaw water treatment works in Preston detected small traces of cryptosporidium - a microscopic bug - in the water.
Samples of water from the treatment works and the wider network show that the traces of cryptosporidium are continuing to reduce.
However, United Utilities said testing would continue and the boiled water notice would not be lifted until they were “absolutely satisfied” that the water was back to the usual standards.
Investigations are also ongoing into how the bug found its way into water at the treatment works.
United Utilities is putting cards through letterboxes of affected homes from today.
Dr. Martin Padley, United Utilities’ chief scientific officer said: “We’d like to thank customers for their immense patience as we work to restore normal service.
“As a precaution, we’d ask people to continue to boil their water for drinking, food preparation and brushing of teeth until further notice.
“The boiled water notice will apply throughout the weekend, and most likely, into the early part of next week. We’ll continue to keep customers informed.”
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