Hundreds of thousands of people in Lancashire have been warned to continue boiling tap water.
United Utilities first issued the advice last Thursday after tests at one of its water treatment plants discovered traces of the microscopic bug cryptosporidium, which can cause sickness and diarrhoea.
It said it was making "good progress" in tackling the problem which it maintained posed a "very low" health risk but the precaution needed to stay in place.
The affected households cover a large area of Lancashire including Blackpool, Preston, Chorley and the Fylde coast.
United Utilities is advising customers to boil their water and then let it cool for all drinking, food preparation and teeth brushing
People can continue to use tap water without boiling for general domestic purposes such as bathing, flushing toilets or washing clothes.
In a statement, the water firm said: "The boil water advice is still in place today in Lancashire as a precaution but we are making good progress and the situation is improving.
"We are very sorry for the continuing inconvenience. Thank you very much to all of our customers in Lancashire for their patience and understanding whilst we try to get things back to normal."
It 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."
United Utilities is to open a number of customer information centres in Preston, Leyland, Chorley, Blackpool and Fleetwood.