Rats could grow to the size of sheep or even bigger as they evolve to fill vacant ecological niches, according to a UK scientist.

The terrifying scenario could become a reality as super-adaptable rats take advantage of larger mammals becoming extinct, geologist Dr Jan Zalasiewicz, from the University of Leicester, predicts.

“Animals will evolve, over time, into whatever designs will enable them to survive and to produce offspring,” Dr Zalasiewicz said.

“For instance, in the Cretaceous Period, when the dinosaurs lived, there were mammals, but these were very small, rat and mouse-sized, because dinosaurs occupied the larger ecological niches.

“Only once the dinosaurs were out of the way did these tiny mammals evolve into many different forms, including some very large and impressive ones: brontotheriums, horses, mastodons, mammoths, rhinoceri and more.”

Dr Zalasiewicz said, given enough time, rats could grow to be at least as large as the capybara, the world’s largest rodent, which can reach 80kg.

“If the ecospace was sufficiently empty, then they could get larger still.”

He said rats in many cases had out-competed many native species on islands, at times driving them to extinction.

“As a result, ecospace is being emptied, and rats are in a good position to re-fill a significant chunk of it, in the mid to far geological future.”

He suspected that rats will have a major influence on the geological future of the Earth and over time were likely to produce “some remarkable descendants”.