In Cheshire, a King Charles spaniel called Kendall collapsed and died minutes after paddling in a lake. In Craigavon, Co Armagh, Bella the labrador puppy died four days after splashing through waterways. A cygnet died in a park in Southampton. Two dogs fell ill after swimming in a reservoir in Cornwall. In Rutland, a beach and aqua park were closed.

Across the UK, from Aberdeen to Hampshire, Edinburgh to Lincolnshire, there have been warnings to stay out of lakes, reservoirs and other waterways after blooms of an organism were discovered. Many people are unaware of the algae which produces one of the most potent biological toxins in the natural world – and to which there is no known antidote.

The British Veterinary Association this week urged pet owners to take particular care around waterways that may be affected by blue-green algae, stressing that it could be particularly harmful to animals even if only a small amount was ingested.

Dogs and other pets exposed to the algae could experience vomiting, diarrhoea, disorientation and seizures, potentially leading to liver damage and death, the national body for vets said.

According to reports, at least four dogs have died in the UK after venturing into to waterways known or thought to have be home to the algae. It has also been causing harm this summer in the US, where the Great Lakes have been particularly badly affected, according to scientists. Three dogs reportedly died in North Carolina this month after swimming in a contaminated lake.

The blooms are caused by cyanobacteria, organisms that date back billions of years, according to Prof Laurence Carvalho, of the UK’s Centre of Ecology and Hydrology. While they contain chlorophyll, the green pigment used in photosynthesis, they are not technically plants.

When the bacteria decay, they release toxins, a turquoise pigment (hence the name) and a potent odour that Carvalho compared to rotting vegetables.

The incidence of blooms is increasing owing to two key factors: nutrients and heat. “The fundamentally important thing is plant nutrients – nitrogen and phosphorus. It’s the pollution of our lakes and reservoirs with treated sewage effluent and agricultural runoff,” Carvalho said.

Most sewage treatment removes organic chemicals and harmful bacteria, he said, “but they don’t remove nutrients, so when that effluent enters our waters they are much more enriched than in natural conditions.”

His team have found that the conditions for the blue-green algae’s growth are further enhanced when the water temperature is above 17C. Thanks to the climate emergency, those conditions are being met more often. “There’s pretty strong evidence that climate change is leading to increased problems, more widespread and intense blooms, right across the globe. We expect with warming that things will get worse.”

One particular strain of the algae, Cylindrospermopsis raciborskii, has been spreading northwards through Europe and has reached Germany. “We are expecting that to probably arrive in the UK in the next few years with climate change,” Carvalho said.

Facebook Twitter Pinterest A warning sign in Grovelands Park, north London, which had a blue-green algae outbreak in 2012. Photograph: Linda Nylind/The Guardian

Cases of human poisoning are extremely rare. In 1989 two soldiers training in canoeing in Rudyard Lake in Staffordshire became seriously ill with atypical pneumonia and others reported abdominal pains, vomiting, diarrhoea, blistering of the mouth and sore throats after ingesting some of the toxins. Carvalho said a number of studies in the US had examined whether there was a link between toxic algae and clusters of motor neurone disease and Alzheimer’s cases close to lakes.

In order to facilitate the reporting of potential cases and speedier public warnings, Carvalho recently led the development of an app called Bloomin’ Algae, which invites people to upload photographs of blooms they are concerned about, which can then be scrutinised by his team of freshwater ecologists (it is frequently confused with duckweed and blanketweed, he says, both of which are harmless).

The app is already widely used by environmental health officers in Scotland, but otherwise only about 40 or 50 of the UK’s 44,000 lakes and reservoirs are regularly monitored for algal conditions.

One location where a user had submitted a photograph for query was Broomfield Park in Enfield, north London. The app judged it to fall into the “plausible or photo not clear” category. A source at Enfield council said its waterways were regularly monitored for toxins and pathogens and it had had no reports of potential outbreaks.

But although the nearby Grovelands Park had an outbreak of blue-green algae in 2012 and still has prominent (if peeling) warning signs, dog-walkers in Broomfield on Friday morning were largely unconcerned.

Julie Clifford, walking her black labrador Bill, said there was plenty of discussion about blue-green algae on the dog-owner message boards she read, but she added: “I’m not concerned because he won’t go anywhere near the water.”