Bristol Water is urging people to stay safe and not swim in the water at spots like Chew Valley or Cheddar Lake this weekend.
The warning follows high levels of blue-green algae being reported across Chew Valley Lake, Litton Lake, Cheddar Lake and Barrow Tanks.
The algae can be toxic to both dogs and humans.
The Head of Recreations at Bristol Water says he has to speak more than 300 people failing to enjoy the lake safely over the past week.
Algae can make the water look bright green or brown tinged, it may also foam towards the surface.
It can often be mistaken for sewage as it can have an earthy smell. But Algae do not affect water supplies as water goes through an intensive treatment process before it reaches your tap.
Bristol Water say they will be actively monitoring the lakes over the weekend, with regular security patrols in place