At least 330 cats died from a mysterious illness investigators believe could be linked to recalled cat food, according to the Royal Veterinary College (RVC).
Vets across the UK have treated at least 528 cases of feline pancytopenia.
The illness causes the number of red and white blood cells and platelets in cats to rapidly decrease, causing serious illness which can often be fatal.
The RVC said that of the cases treated at their own hospitals or by vets, there was a 63.5% mortality rate.
The true number of cases could be higher, it warned, because many cats who become ill may not be taken to a veterinary practice.