Those visiting Wollaton Park have been warned to avoid the lake following a blue-green algae bloom.
The Cyanobacteria - a bacteria often referred to as algae due to its appearance - releases toxins and becomes more abundant during hot weather, and this can be poisonous for both dogs and humans.
Charities, including the Blue Cross, say if an animal ingests the toxic water during a bloom, it can prove fatal in the space of just 15 minutes.
Facilities at the park opened back in May and it is regularly used by walkers, particularly during the warm weather.
In a customer notice issued on social media, Wollaton Park said: "Our lake is beautiful to stroll round with the glorious weather, though take extra care when walking dogs, as it currently has blue green algae bloom in the lake, due to the especially hot weather.
"It contains toxins that are poisonous to dogs and humans, so we advise that dogs should be kept on a lead, away from the lake water."
Animal welfare charity the Blue Cross says symptoms can include vomiting, seizures, unconsciousness, confusion, drooling or breathing difficulties.
It says: "If your dog shows any of the following signs after drinking from, or swimming or paddling in water, contact your vet immediately and tell them you are concerned about blue-green algae.
"There is no antidote for the toxins produced by the bacteria, but if caught early enough, your vet will likely try to make your dog sick and attempt to flush the toxins from the body before they take hold.
"Sadly, blue-green algae poisoning often eventually causes fatal liver failure.
"These symptoms are commonly seen with other illnesses too, which are often less serious, but you should always call your vet if you are worried your pet is sick."
