Wessex Water has issued a boil water notice to 250 properties in the Bratton Seymour, Shepton Montague, Yarlington, and Maperton areas of Somerset advising customers to boil their water before using it.
The advice was issued yesterday evening as a precautionary measure after Wessex found the water supply was "not of its usual standard."
Customers in up to 250 properties will need to boil their tap water before using it for drinking, washing food, cooking, cleaning teeth or for medical purposes. Water directly from the tap can still be used for washing, bathing and flushing the toilet.
The water company has written to all customers affected and provided bottled water for each home.
The boil water notice will remain in place until at least Friday 11 September and customers affected will need to boil water until they receive an "all clear" notice through their door.
Nigel Martin, water supply general manager, said:
"As a responsible water company we are committed to supplying customers with water of the highest quality and it is through one of our vigorous tests that we identified a water sample didn't meet our standard. “
"Therefore, as a precautionary measure we are asking some customers to boil their water and have supplied bottled water to those affected. “
"Once we are completely satisfied that the water quality is back to its normal high standard we will inform customers that they no longer need to boil water.”
A spokesperson for Wessex Water said there was nothing to indicate the presence of cryptosporidium in the water sample which had identified the potential problem.