In Scotland, an estimated 87% of pets are being fed "deadly diets" putting them at risk of an early grave, the study concluded.
PDSA senior veterinary surgeon Elaine Pendlebury said: "Pet obesity poses not only major health risks such as diabetes, arthritis and heart disease, but sadly also means daily misery for millions of pets who are feeling the strain from carrying too much weight.
"Vet practices across the UK see the consequences of pet obesity every single day such as obese dogs unable to enjoy regular walks due to exhaustion, fat cats that can't jump or play, and rabbits so hopelessly overweight they can't clean themselves properly. And then there are other obesity-related health conditions which can mean pets don't lead the long and happy lives they should."
Dr Alex German, an animal obesity specialist at the University of Liverpool's veterinary school, said pet obesity was "entirely preventable" and could be reversed with "veterinary supervision and owner dedication".
The PDSA has now launched its annual animal slimming competition to find Britain's fattest pets and help them get fit.