Several pet food brands contain beef, chicken and pork which is not listed on the ingredients - say researchers at the University of Nottingham.
They found DNA from the three animals in 14 leading brands of dog and cat foods, despite the meats not being mentioned on the products' labels.
Two types of pet food which claimed to contain chicken, actually contained more pig or cow meat, and five samples "with beef" contained more pig DNA.
It may be a surprise to shoppers to discover that prominently described contents such as 'beef' on a tin could, within the guidelines, be a minor ingredient, have no bovine skeletal muscle (meat) and contain a majority of unidentified animal proteins.
Researchers say no regulations have been broken but that pet owners are not getting the products they expect and the current system could cause religious offence.
Full disclosure of animal contents will allow more informed choices to be made on purchases which are particularly important for pets with food allergies, reduce the risk of product misinterpretation by shoppers, and avoid potential religious concerns.