The PDSA, which offers free and reduced price veterinary treatment, is opening up its Pet Fit Club awards again this year.
Successful applicants are placed on a six-month diet and exercise programme, with the pet losing the largest percentage of excess weight crowned Pet Fit Club Champ.
Previous entrants have ranged from dogs and cats, to rabbits and rats.
Now in its 15th year, it has previously helped 143 pets lose a combined 513kg (80st 12lb) - equivalent to the weight of a polar bear.
PDSA vet nurse Nina Downing said pet obesity remained a "huge problem" nationally, with up to half of animals seen by vets thought to be overweight.
Carrying too much weight is a serious health issue, with research showing it can cut life expectancy in dogs by up to two-and-a-half years.
Animals are also more likely to develop conditions like arthritis and diabetes and aggravate existing problems, like heart disease.
It can also lead to complications for dog breeds like Pugs and French Bulldogs, who already struggle with breathing because of their flatter faces.
Ms Downing added: "We know that we are a nation of animal lovers but unfortunately a lot of pet owners show their love and affection using food."
She said tackling the issue started with owners acknowledging the problem, and then making sure their precious pets were getting the correct diet and enough exercise.
"Most pets would be just as happy with extra attention, an extra walk or playtime, than food," Ms Downing added.
She said human food "treats", often packed with calories, salt, sugar and fat, and portions that were too large could be very bad for pets.
"To a medium-sized dog, eating three cubes of cheese is like a person eating two chocolate bars," she added.
Ms Downing said: "They should be fed a complete pet food suitable for their age and weight, with only occasional healthy treats, such as a few slices of carrot for dogs and a small piece of white fish or meat for cats."
She said it was never too late to make a change and urged owners to enter their overweight pets.
Among this year's entries is cheese-loving four-year-old female Jack Russell Skylar, who is an eye-watering 11.5kg (22lb), which is more than double its ideal weight.
Owner Mandy Hannigan - who admitted to spoiling the rescue pooch with sausages, ham, cheese and crisps - has decided to take decisive action.
Ms Hannigan, from Solihull, West Midlands, said: "Skylar eats absolutely everything, and it's very difficult to say no when she gives you those puppy-dog eyes.
"I'm guilty of feeding her human foods, especially her favourite snacks like Doritos and Quavers - she loves them.
"Every morning she'll eat the crusts of my toast and, when I'm eating my dinner she sits there waiting for half of whatever I'm eating."
She revealed Skylar also tried to eat food belonging to their other dog, Dodger.
"I want to help her lose weight for the sake of her health and happiness, so I've entered Skylar and we are hoping to be selected," Ms Hannigan added.
More information can be found at pdsa.org.uk/petfitclub or by ringing 01952 797246.
Entries must be in by March 8.