South West Water (SWW) is to be told by Cornwall Council to urgently address the impact of waste water discharges.
The council voted unanimously for action on discharges into local rivers, bathing waters and shellfish harvesting areas, and to fund investment.
It also called for such work to be funded through the curbing of bonuses and dividends rather than passing costs on to consumers.
South West Water said it had reduced such spills this year.
In the motion, approved at a Cornwall Council meeting on Tuesday, the authority said "every river in Cornwall and across England is now polluted beyond legal limits and that chemical pollution is mostly caused by sewage discharges from water companies and run-offs from farms through poor nutrient and livestock management".
The council said the Fal and the Camel estuaries were "particularly threatened by further sewage discharges and agricultural run-off, with both having shellfish harvesting areas now at risk of declassification".
The motion said the council would write to SWW's chief executive, as well as the Secretary of State for Environment, Food and Rural and the chairperson of the Parliamentary Environmental Audit Committee to advocate for greater enforcement of "existing regulatory powers" for both sewage discharge and what farms put into water courses.
SWW said it knew that "reducing the use of storm overflows is an important issue for customers, as is the health of our rivers and seas - and it is one of our key priorities too".
It said it said it had reduced spills "in this year's bathing season" by 50% on 2021, and the duration of those spills by 75%.
It added that its WaterFit programme would see it "reduce our use of storm overflows and reduce our impact on rivers by one-third by 2025" and it would "continue to work closely with partners across the region, including Cornwall Council".