Toxic chemicals found in pet flea treatment are devastating wildlife in rivers, parks and special conservation areas and the government should take urgent action to limit their use, according to a study.

Chemicals that are banned for use as pesticides but still used in flea treatments are causing potentially irreversible harm to aquatic life as well as decimating birds and pollinators, according to the study published on Thursday.

It found that flea treatment chemicals fipronil and imidacloprid have also been implicated in lower cognitive and adaptive scores in children with autism and was ranked by the Environment Agency as being the chemical with the highest risk to human health in English waters.

The assessment by ecologist Matt Shardlow, who carried out one of the first studies into the impact of flea treatments nine years ago, says urgent action is needed to prevent these chemicals doing more damage.

“The more you look at this, the worse it gets,” said Shardlow from the Wildlife and Countryside Link. “The level of new alarming information showing the scale of damage that is being done by these chemicals is stark.”

Last month the Guardian reported on calls for restrictions on pet flea treatments after research found songbird feathers were widely contaminated with chemicals that can damage their brains and kill unborn chicks. A previous study found chicks were being killed by high levels of pesticides in the pet fur used by their parents to line their nests.

The analysis found irreparable damage may already have occurred to rivers, sites of special scientific interest and National Parks. It highlighted the Broads in Norfolk, where all the main rivers entering the park are heavily contaminated with flea treatment toxins, and where Natural England is concerned about a 90% decline in dragonfly numbers.

It found that average levels of fipronil and imidacloprid were high enough in English rivers to “reduce aquatic life”. In a fifth of cases levels were found to be sufficient to cause a 30% reduction in “associated bird populations”.

“We are seeing not just impacts in rivers but in public parks where dandelions were so heavily polluted that it is likely to harm caterpillars and pollinators,” said Shardlow.

Flea and tick treatments, which are commonly used on the nation’s dogs and cats, are the main route through which chemicals such as fipronil and imidacloprid enter the environment, the review said.

The substances can enter through the sewage system from when treated animals or contaminated clothing or bedding are washed. They can also get into the environment when dogs go swimming, when pets rub against vegetation, or shed fur outside.

The government announced an eight-week consultation on banning UK pet owners from buying the treatments for cats and dogs over the counter, but Shardlow said that did not go far enough.

He said even if that went ahead these products would still be widely available in pharmacies, supermarkets and vets, as well as online.

In his report Shardlow calls on the government to undertake an urgent environmental risk assessment to understand the scale of the problem, adding the “only sensible option” would be for the government’s Veterinary Medicines Directorate to review, suspend and ban the flea treatments containing fipronil and imidacloprid.

“The government has simply not done what a proper environment regulator should do and look into the new evidence,” he said.

A spokesperson for the Department for Environment, Food and Rural Affairs said the government was “committed to restoring nature and cleaning up our rivers whilst upholding the highest animal welfare standards”.

“This consultation is another important step towards reducing forever chemicals in our waterways and protecting the nation’s wildlife. We encourage Wildlife and Countryside Link to respond to the consultation.”

A spokesperson from the Veterinary Medicine Directorate said: “We recognise the benefits of fipronil and imidacloprid and the role they play in protecting pets and people from parasites and the diseases they carry. However, these substances are entering our watercourses and could be contributing to wider environmental impacts.

“We want to hear as many perspectives as possible in this consultation to help us maintain appropriate market availability while also protecting our animals and the environment.”