There are fears a new form of 'mutant rat' which cannot be killed by regular poisons is spreading across the UK.
The rats, which have been spotted in Kent, the West Country and Sussex, look the same as normal rats but eat toxic pellets "like feed".
In 2012, there were also reports of the so-called 'super rats' being spotted in Oxford and Berkshire, leading to fears they could spreading across Britain.
Pest controllers are now seeking permission to use stronger poisons against the rats in a bid to control the rise in numbers.
The British Pest Control Association's Richard Moseley told the Metro: "Normal rats are being killed off by poison, so these resistant species are taking their place - it's only natural that their numbers are expanding.
"But they're being found further afield than previously anticipated.
"They eat poison like feed, you might as well be leaving out grain for them."
It is believed a naturally-occurring mutation of genes is the most likely reason for the rats becoming immune to standard-strength poisons.
Dr Dougie Clarke, of the University of Huddersfield, said: "Poisons such as the ones you buy in B&Q are not effective against them, nor are those used by pest control experts.
"There are obviously health concerns and worries about the bacteria they carry, such as salmonella.
"They carry a lot of diseases, including Weil's, which has been linked to deaths. They also chew on electrical cables."
Councillors in Home Counties such as Berkshire and Hampshire previously requested permission to use stronger poisons in a bid to tackle the new strain of rats which plagued the area.
Requests to use stronger rodenticides such as Brodifacoum and Flocoumafen had been denied by the Health and Safety Executive over fears it would also damage the environment and poison others and wildlife too.