Copenhagen officials have issued warning to residents to stop drinking tap water Saturday after samples collected found trace evidence of E.coli contamination.
"The tests... show that parts of Norrebro and another central Copenhagen neighbourhood have been affected," the water distribution agency, Kopenhamns Energi, said in a statement.
It said the unusually high levels of E.coli were detected late Friday during routine testing of the city's tap water.
It urged residents to boil tap water for two minutes before drinking it and added that further test results were due on Sunday.
According to the Berlingske daily's website, the contamination may have been caused by unclean rain water seeping into the municipal system.
Drinking water contaminated with the bacteria can cause diarrhoea and vomiting but no cases were reported Saturday.
Earlier this year, an outbreak in Germany of a string of the bacteria killed more than 50 people.