Less than a quarter of owners said they referred to food labelling advice
Nine out of 10 pet-owners are unconcerned about their animals' size despite an increase in overweight pets, a survey suggests.
Some 62% of dog-owners and 72% of cat-owners believed they could do little or nothing about their pets' weight.
More than 2,100 UK households were surveyed on behalf of the Pet Food Manufacturers' Association (PFMA).
A report by the People's Dispensary for Sick Animals in 2008 claimed one in three UK dogs was overweight.
However, in the TNS survey for the PFMA, eight out of 10 owners believed their pet was the correct weight.
But when shown pictures of animals of varying weights, just 33% of dog-owners and 23% of cat-owners said their animal resembled the "normal weight" image.
PFMA chief executive Michael Bellingham said perceptions of pets' correct size were "seriously out of kilter with reality".
"Pet obesity has a serious, and sometimes fatal, impact on the health and welfare of our pets.
"The figures are alarming and the problem won't go away without a fundamental shift in owners' attitude to feeding their pets," he said.
"It is also worrying that most owners are not aware of the problem, because they are unable to recognise a fat pet in the first place.
More than a third of owners who admitted having an overweight pet said they fed it too many treats, while a similar proportion blamed lack of exercise.
Only a quarter of respondents said they had ever sought advice on their pets' weight.