A Swedish pest control company has confirmed the discovery of sewer rats with an immunity to commonly used anti-rodent poisons.

According to extermination company Anticemex, poison-resistant rats have been found in four separate locations in Sweden, confirming long-held suspicions that anti-rodent agents were increasingly ineffective.

The discovery is the result of 80 random tests performed by Anticemex, who revealed the immune rats were found in the cities of Kristianstad, Linkoping, Vaxjo and Uppsala.

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Anticemex’s pest control expert Hakan Kjellberg said: “It may have been rat poison, but also chemicals in their immediate environment that have caused the genetic makeup in their body to change”.

Studies into poison-resident rats have previously been carried in Scandinavia, with confirmed cases in both Denmark and Norway.