Hundreds of cats may have died because their owners mistakenly treated them with anti-flea products intended for dogs, a study suggests.
The chemical is toxic to cats
The Veterinary Poisons Information Service found that one in 10 cats referred to it had died after being exposed to permethrin.
The chemical is used in flea treatments for dogs but is very toxic to cats, said Alex Campbell of VPIS.
VPIS wants clearer warnings to be displayed on canine treatments.
Mr Campbell said the substance was present in many products, but in very low concentrations.
If accidentally applied to cats they can show "severe clinical signs" and need two or three days of intensive veterinary treatment if they are to survive.
"You'd find it in ant powders and a few things like that, but in those sort of products it's in very low concentrations, so it doesn't usually cause problems in either cats or dogs," he told BBC Radio 5Live.
"However, it is occasionally used in spot-on flea treatments for dogs and if you accidentally apply these to cats, or you've treated your dog and your cat comes into contact with the dog, and actually manages to groom some of it off or whatever, then potentially the cat can get severe clinical signs.
"This substance is very toxic to cats."
In a study of 286 cases in which canine spot-on permethrin preparations had been used on cats found that 97% showed signs of poisoning.
Around 90% displayed symptoms of twitching and convulsions, with one in 10 dying or having to be put down.
However, Mr Campbell said poisoning may be more widespread as not all vets report every case, nor do they all use the VPIS, which is part of Guy's and St Thomas' NHS Foundation Trust.