Residents in parts of Utrecht and surrounding towns have been told to boil tap water before drinking it, after the discovery of bacteria in the local supply.

Drinking water company Vitens said tests had detected enterococci bacteria in a storage facility in Utrecht. The source of the contamination is still being investigated, Vitens said.

The bacteria occur naturally in human intestines and are generally harmless to healthy people, but they can cause infections in those with weakened immune systems or other health issues.

The boil-water advisory affects around 125,000 households in parts of Utrecht, Bilthoven, Bosch en Duin, De Bilt, Bunnik, Groenekan, Maarssen, Oud Zuilen, Tienhoven, Westbroek and Zeist. Residents are advised to boil tap water for three minutes before using it for drinking, brushing teeth, or washing food.

Vitens said that tap water is still safe for showering and washing hands, and can also be used for cooking. However, drinks such as coffee, tea and lemonade should only be prepared with boiled water.

Supermarkets in Utrecht saw long queues on Saturday morning as shoppers stocked up on bottled water. Some stores reported empty shelves, while others still had limited supplies available.

Vitens said it had contacted all affected customers by text message or email and personally called key institutions, including hospitals. “We also urge people to share this advice with neighbours, friends and on social media,” a spokesperson said.

The company could not yet say when the advisory will be lifted. “Two consecutive clean test results are needed,” the spokesperson said. “We expect to update residents by Tuesday at the latest.”

Water institute KWR researcher Roberta Hofman-Caris told broadcaster NOS.contamination can occur in several ways.

“Sometimes a bird or mouse ends up in a reservoir, or a small leak develops during maintenance,” she said. “The threshold for issuing a boil notice is quite low — the presence of enterococci indicates faecal contamination, and for safety reasons the water must then be boiled.”