Some 40,000 residents of Sundsvall in northern Sweden may have been exposed to contaminated water following the discovery of coliform bacteria in the city's mains water supply.
"It's serious, but all routines have worked," said Tomas Östergren at the environmental office in Sundsvall to the local Dagbladet daily.
The city's authorities have announced that the mains water system will be chlorinated in order to protect inhabitants against infection.
"It can take up to a day before the chlorination reaches everywhere," said Uno Schön at water firm Mittsverige Vatten to the newspaper.
The discovery of the coliform bacteria, which are not necessarily a cause of serious illness but can indicate the presence of other pathogens, was made on Friday morning.
The infection is reported to cover the entire Grönsta water catchment area which supplies central Sundsvall, home to some 51,000 people.
It remained unclear on Saturday just how many people may have been drinking contaminated water before the bacteria were discovered.
Nearby Östersund suffered a Cryptosporidium bacterial outbreak in 2010 which left as many as 30,000 people suffering from stomach problems. In July 2013 the municipality was charged with environmental crimes over the outbreak.
With regards to the Sundsvall there are currently no criminal suspicions of wrong doing and the cause of the infection to the municipality's water supply is under investigation.