Up to 6,000 households in Norfolk are being advised to continue boiling drinking water after concerns that a parasite may have entered supplies.
The boil water notice is set to continue into next week
Tests on water supplies on Saturday confirmed cryptosporidium, which can cause diarrhoea, may have entered the drinking water supply.
The affected areas are North Walsham and North Walsham Road, Felmingham.
Anglian Water said it was now likely that the restriction notice would be in place until early next week.
Drinking water and water used for food preparation should be boiled first.
The normal supply can still be used for washing, bathing and flushing the toilet.
Bottled water is being supplied to vulnerable people.
The cryptosporidium parasite can cause diarrhoea, stomach cramps, upset stomach and a slight fever.
However, illness tends to strike older people and children.
The parasite can easily be filtered out of water supplies by Anglian Water's own equipment due to its size.