Rats are becoming resistant to existing poisons, according to pest control experts who warn that the government must allow the use of more powerful pesticides if the burgeoning rodent population is to be contained.

Amid concerns that the number of rats is soaring out of control, the British Pest Control Association says it has identified two unnamed towns in Berkshire where it claims the local rat population is now almost completely immune to existing poisons. Oliver Madge, chief executive of the association, said: "We are very concerned about this from a public health point of view."

The trade body will hold urgent talks with the Department of Health this week about introducing measures that it claims are needed to halt the spread of the rodents, thriving after years of mild winters and wet summers. Britain's rat-catchers want the law changed so that two powerful rodenticides, brodifacoum and flocoumafen, which the law only allows to be used indoors, can be applied outdoors. Other EU countries let pest controllers use the more powerful poisons outdoors but the UK believes this could endanger birds and pets.

In the worst-hit cities, such as York, reported infestations have more than doubled in the past 12 months. Exeter council reported a 66% rise in vermin call-outs last year, while there was a 40% rise in Salford. There are concerns that some water companies have dispensed with putting poison down sewers.

Madge said that recycling food and water was also playing a part. "We are encouraged to recycle, but the rats tunnel under compost and they find it's nice and warm and they have food."