More than 30 doctors from UKC Ljubljana, Slovenia's largest hospital, wrote a letter in opposition of a new sewage project.
They said the project was a potential threat to drinking water.
The plan is to carry waste water from Medvode and Vodice north-west of Ljubljana to a central waste water treatment facility in Zalog. It will also pass through the boroughs of Črnuče and Brod.
The doctors warned that in the event of an earthquake or extreme weather events due to climate change, sewer leaks could occur, contaminating Ljubljana's most important drinking water supply system.