Pet owners who overfeed animals have been warned they could cause as much suffering as those who starve them.
Being only slightly overweight can cause health problems
Four out of five vets said they were seeing more overweight and obese animals, according to 143 practices who were contacted in a survey.
Overweight pets risk serious health problems, the Royal Society for the Prevention of Cruelty to Animals (RSPCA) warned.
Podgy pet "fat clinics" have been set up by more than three quarters of vets.
Pet obesity can cause arthritis, high blood pressure, poor liver function and diabetes, amongst other conditions.
Inspectors saw several cases last year where animals including dogs, horses, a donkey and even a parrot suffered from being overweight or obese.
In one case, a dog was so heavy it could not move after being fed on a diet including full English breakfasts.
The dog had to be carried to the vet by the RSPCA on a makeshift stretcher. Its owner was banned from keeping pets for life.
A separate but related survey of 2,300 pet owners in England and Wales by the organisation suggests that one in three owners believe their pet is overweight.
More than 80% of the vet practices thought pet owners were surprised when told their animals were overweight.
RSPCA vet David Grant said. "An animal only needs to be slightly over its ideal body weight for health problems to begin."
A leaflet that warns pet owners of the dangers of letting their animals get overweight has been launched as part of RSPCA Week.