The British Pest Control Association chief executive Simon Forrester said "Reports of poison-resistant rats have been increasing in recent years and it seems likely that there'll be a further surge in numbers.
"The trouble is that people who try to treat problems themselves are likely to be making the problem worse.
"The rodents have become resistant and, in some cases, immune to off-the-shelf poisons to the point where they're actually feeding off the toxic pellets, which means their size and strength is increasing.
"Stronger rodenticides can be more effective, but most are subject to strict legislation and must only be used by professional pest controllers.
"So it has become very important to make sure infestations are treated by experts in the field."