Nearly nine in 10 pets in Scotland are at risk of an early death because of bad diets which can include takeaways, crisps and cakes, according to research.
More dogs, cats and rabbits than ever before are being fed "potentially life-threatening" high-calorie meals, the PDSA found.
If coupled with little or no exercise, their life-spans could be cut short by preventable obesity-related conditions such as arthritis, diabetes and heart disease, the veterinary charity warned.
Across the UK, an estimated 18.5 million pets are regularly being fed inappropriate diets that are not as healthy as they should be, the PDSA animal wellbeing (PAW) report found.
Of that number, about 13.5 million animals are "treated" to fatty or sugary treats and junk food.
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.