Several residents around the inner Oslofjord have received a text message in recent days in which they are advised to bring the drinking water to the boil.
"There is a discovery of bacteria in the drinking water at a test point in the area, so what we are doing now is that we keep trying to find the source," said communication advisor Tone Spieler at the Nettavisen Water and Wastewater.
The warning was issued for the first time before the weekend, but the problem is still not resolved.
"We have taken new samples during the weekend and unfortunately still have discoveries of contaminated bacteria, so we must maintain the cooking advice in the area until we are sure that the water is clean on the municipality's website.
The Council applies to people in the area west of Mosseveien from and to Kongshavn and to the south to Ulvøya, as well as the islands of Bleikøya, Sjursøya, Ulvøya, Ormøya, Malmö and Malmøykalven.
"If the water is boiled, it is not dangerous, but the water must boil for a minute," Spieler emphasizes.
The municipality is working on finding the cause of the pollution, but Thursday will not have an answer to new tests and possibly increase the cooking advice.