More than 18 million pets are at risk of early death because of poor diets which often include takeaways, crisps and cakes, according to a report.
Unprecedented numbers of dogs, cats and rabbits are fed "potentially life-threatening" meals which can lead to arthritis, diabetes and heart disease, veterinary charity PDSA has found.
The "deadly diets" have left animals unable to walk, play or even clean themselves, it said.
While an estimated 18.5 million pets are fed inappropriate diets, some 13.5 million animals are "treated" to fatty or sugary treats and junk food, the PDSA Animal Wellbeing (PAW) Report revealed.
Elaine Pendlebury, PDSA senior veterinary surgeon, 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".
PDSA has now launched its annual animal slimming competition to find Britain's fattest pets and help them get fit. Owners can enter their pets at www.pdsa.org.uk/petfitclub. Entries close on Friday April 12.
YouGov surveyed nearly 4,000 dog, cat and rabbit owners; 466 vets and vet nurses; and 553 children for the report this month.