Genetic mutations have produced "super rats" with DNA that protects against common pesticides, said Robert Smith, a researcher at the University of Huddersfield.
"Natural selection means that when you have a rat population in your town, poison will kill the ones that aren't resistant, the ones that survive may have the gene, they then have babies who can receive the gene themselves," Smith told The Daily Telegraph in a story published Friday.
Exterminators are using traps, guns and dogs to try to keep the rat population under control across Britain. The British Pest Control Association said the government needs to approve more powerful pesticides or the rat population will swell to greater numbers during the summer.
"It is a warning of things to come," Smith said.