Farmers have been advised to beware of new hotspots of 'super rats' that are resistant to rodenticides, a report says.
In East Anglia and West Yorkshire, the report identifies the L120Q gene for the first time, responsible for the most severe form of resistance.
This gene renders first generation anticoagulant rodenticides and two of the second generation groups ineffective.
It is widespread across the whole of central southern England and also found increasingly outside that area.
Another serious concern highlighted in the University of Reading report is that three different types of resistant rats are now found in West Yorkshire and on the Anglo-Welsh border. There is almost a complete lack of data from central England.
Rodenticides are pesticides that kill rodents. Although rodents play important roles in nature, they may sometimes require control. They can damage crops, transmit disease, and in some cases cause ecological damage.
According to co-author of the the report, Dr Colin Prescott, it is not known whether resistance is present in the central region or not.
"The few samples we do have show that rats are mostly susceptible to anticoagulants, but we need many more to be confident of this,” he says.
Although the study covers mice as well as rats, only nine new mouse samples were sent to Reading University in 2018 and these continue to show very high incidence of resistance.
In all, 89% of mice tested were highly resistant to anticoagulants. The Campaign for Responsible Rodenticide Use (CRRU) chairman Dr Alan Buckle says more samples from both rats and mice are desperately needed.
"We can only manage the spread of resistance when we know where it is," he says. "With so few mouse samples and the void of rat data in the centre of the country, we are a long way from that.
"Presently we have the worst of both worlds. Farmers, pest control technicians and gamekeepers are using products that are ineffective in places where rodents are resistant; and they are using unnecessary, resistance-breaking products where there is no resistance. Only more samples can solve this."
Dr Buckle adds that all rodenticide users are obliged to follow the CRRU Code of Best Practice, which includes monitoring the results of control treatments.
"If this suggests that rodents are surviving well-implemented control programmes, this may indicate the presence of resistance," he suggests.
"Those struggling to control resistant rodents can call on the services of a reputable professional pest control company."