As a result one in three dogs - 2.5 million - and one in four cats - two million - are overweight, according to a study by People's Dispensary for Sick Animals (PDSA).
This puts about 10 million pets at risk of disease and early death by eating fatty scraps.
The PDSA, which provides free veterinary care to needy owners, issued the warning as it launched its annual Pet Fit Club dieting contest.
Her owner admitted she spoiled the former stray with treats to make up for her awful start in life.
PDSA senior veterinary surgeon Elaine Pendlebury said: "Sadly, seeing morbidly obese pets is now an everyday occurrence in vet practices across the UK; it is one of the biggest welfare concerns facing the nation's pets."
"Pet obesity significantly increases the danger of developing major health problems such as diabetes, arthritis and heart disease and can also bring about the onset of these chronic diseases much earlier," the daily quoted her as saying.
"PDSA's findings are very worrying and demonstrate that diet remains one of the most misunderstood welfare needs for pets. Obesity is one of the most pressing health issues affecting companion animals. PDSA's work in this area is hugely successful," said Dr Philippa Yam, leading animal obesity expert at the School of Veterinary Medicine, University of Glasgow.
