The RSPCA has called for better measures to warn pet owners of the dangers posed by algae after the death of a dog which is thought to have swallowed the organisms while swimming in Lake Windermere.
The Taras family said the death of their cocker spaniel, Holly, last weekend could have been prevented had there been signs warning that the lake could contain blue-green algae, which produces a potentially fatal toxin.
Their call for measures to be put in place have been backed up by the animal welfare charity.
"As a dog owner, I would be incredibly concerned and I would like there to be some sort of information, if it was thought the lake could contain something toxic," said a RSPCA spokesman.
The spokesman added: "Warning signs would help because some people like to let their dogs off the leash when they are in the countryside. Most people are sensible and don't want to cause problems any more than they would want harm to come to their family pets, so it is about giving them the information to make the right decision."
The Environment Agency also called on people to be vigilant after the death of the dog, the latest to die after ingesting the organism. A greyhound and a lurcher died last year after swimming in Windermere.
And, last September the UK's biggest swimming competition, the Great North Swim, was cancelled due to the prevalence of blue-green algae in Windermere.
Tests had suggested the lake was clear but the death of the Taras' dog casts doubt on that conclusion.
Mr Taras said he had have never heard of blue-green algae. He added: "I am shocked there were no signs anywhere warning us about it. People come on holiday here with their pets and children and they go into the lake and I think that if there is a risk of coming into contact with blue-green algae people should be warned about it. To watch a dog die of it is horrendous. It is the worse thing I have ever seen."