Around 500,000 pets are affected, ranging from cats and dogs to rabbits, rats and even budgerigars.
Like humans, their pets have also been suffering from obesity symptoms such as low energy levels, breathing problems, arthritis, asthma, diabetes, liver and heart disease and poor fertility.
The People's Dispensary for Sick Animals (PDSA) senior veterinary surgeon Sean Wensley said the pets' weight gain could be down to their owners giving them fatty and sugary snacks with little knowledge of what harm they were doing.
"Others that have been mentioned are crisps, curry, pizza, ice-cream, cake, and sweets," Telegraph quoted him as saying.
He said that a single biscuit or crisp for a pet could be equal to an entire packet for a human.
Moreover, the Guinness Book of Records has withdrawn its listings for heaviest animals over worries that some owners may be overfeeding their pets to gain recognition.