Pesticides found in dog flea treatments could be harming wildlife in Hampstead Heath's ponds, a study has found.

The Times newspaper has reported that flea and tick treatments applied to dogs have been detected in three of Hampstead’s ponds.

Researchers from Imperial College London found these waters were contaminated with pesticides that are harmful to aquatic wildlife.

The two pesticides, imidacloprid and fipronil, were banned for agricultural use in 2018 due to their high toxicity to bees and other insects, but remain in use for flea and tick infestations in cats and dogs.

The chemicals are increasingly being used as a preventative treatment, regularly applied to pets by owners keen to avoid an infestation, but wash off when dogs swim.

Scientists have now raised concerns about the level of these chemicals reaching the natural environment through household wastewater and dogs swimming in waterways such as rivers and ponds.

A study published in the journal Science of the Total Environment found the chemicals are washing off in ponds where dogs are allowed to swim in Hampstead.

There are 18 ponds on Hampstead Heath, including three bathing ponds for people. There are three ponds where dogs are allowed to swim in designated areas, although owners sometimes allow dogs to swim in restricted ponds.

The researchers from Imperial, the University of Sussex, the London School of Hygiene and Tropical Medicine, and the Royal Veterinary College, tested three ponds where dogs are allowed to swim, three where dogs are not permitted, and six streams that feed these ponds.

In the ponds where dogs swim, they found 309 nanograms of imidacloprid and 32 nanograms of fipronil per litre of water—both more than 20 times higher than the level deemed harmful to invertebrate organisms.

None of the chemicals were detected in ponds where dogs are banned, but low levels of chemicals were found in a stream connecting a dog pond and a human bathing pond.

Prof Jeff Waage, a member of the Heath and Hampstead Society, said chemical levels in the ponds used for human bathing were not measured.

The researchers said that the pesticides are "absent from or at most at very low levels" in the ponds where people swim.

A survey found 86% of 101 dog owners were unaware of the environmental impact of the pesticides.

Dr Leon Barron, from Imperial’s School of Public Health, said: "This work adds to the growing body of evidence that will hopefully lead to changes in prescribing practices and advice given to pet owners.

"But as the public can buy these products without restriction in supermarkets, we need to raise awareness among dog owners that while these chemicals are useful treatments, using them preventatively may create other serious issues such as environmental pollution and pest resistance."