A blue green algae outbreak at Ashby Ville pond in Scunthorpe has been confirmed as toxic by the Environment Agency.
Visitors to Ashby Ville Nature Reserve on Mortal Ash Hill are being advised not to go in or the near water, or to let their dogs go in the water.
North Lincolnshire Council officers have put up warning signs at the nature reserve and have been on site every day to speak to visitors about the dangers of going in the water.
The council now has a warden on site seven days a week to speak to visitors and the algae has been reducing at the pond.
Cabinet member for Safer, Greener and Cleaner Places, Councillor Ralph Ogg, said: “As soon as we were made aware of what looks to be blue green algae in Ashby Ville pond we contacted the Environment Agency to investigate.
“Now that they have confirmed the strain is toxic, it is important that people and animals don’t come into contact with the water or enter the pond.
“Despite the warning signs and our officers on site speaking to visitors, we have still had people swimming in the pond at Ashby Ville. It is important that visitors do not go in the water.”