Two 2ft long rats were caught in a factory in the East Midlands, Rentokil has confirmed.
The pest control agency said the animals were caught within the last four weeks and were double the normal size of a brown rat.
It said access to a plentiful supply of food had allowed the rats to grow to an abnormally large size.
The news emerged following reports in the Sun newspaper that a 30in (76cm) rodent was killed on a Bradford estate.
Rentokil spokesman Malcolm Padley said: "They were both around 2ft and it's fair to say our technicians were very surprised at the size.
"It goes to show that where there's a food source, these rodents will grow big".
There had been suggestions that the animals could be coypu, a plant-eating semi-aquatic rodent originally native to South America.
However, chief executive of the Mammal Society, Marina Pacheco, confirmed this was not the case.
She said the rodents did not have the distinctive orange teeth seen in the species.
A nationwide cull of coypu is thought to have wiped the animals from the wild 20 years ago.
It is illegal to own one unless it is for exhibition or scientific research, in which case it must be licensed.