Test samples have revealed high levels of toxins in a North Yorkshire lake where three dogs died suddenly.
The water at Burton Riggs nature reserve at Seamer, near Scarborough, tested positive for Cyanobacteria.
The toxin, also known as blue-green algae, can prove fatal for animals. It can cause sickness and muscle pain in humans.
Yorkshire Wildlife Trust, which owns the reserve, urged people to stay out of the water and keep their pets away.
Scarborough Borough Council took water samples which revealed the presence of blue-green algae after three dogs died at the lake in June.
Jonathan Leadley, head of communications at the Yorkshire Wildlife Trust, said: "It isn't a huge surprise to us as this is a naturally occurring event in water bodies across the world."
Cyanobacteria usually only becomes toxic after prolonged spells of hot weather when water levels drop and an algal bloom forms.
Mr Leadley said: "We haven't had those conditions recently and all the tests prove is the presence of the algae not whether it is blooming."
Further tests are taking place to try and firmly establish what caused the death of the dogs.
Mr Leadley added: "We would please ask dog owners to keep their animals out of the water until we have this pinned down."