Fears that a new breed of "mutant" rat may be plaguing the Houses of Parliament are growing on reports that the vermin are spreading across southern Britain.
The new strain of rat, which have grown immune to standard poison, have been spotted in Sussex, Oxford and the West Country and are now believed to have been seen scurrying around the feet of politicians in London.
The government is also reported to be spending around £6,000 a month on pest control at Westminster, according to a Freedom of Information request.
There are around 40 sightings of the rodents in the Parliament a month, up from just 10 a month in October 2010.
Officials played down talk of an infestation, insisting many of the rodents sighted were merely "large mice".
A Westminster source told the Daily Star that the Houses of Parliament are an "ideal" place for rats.
"You can see from the pest control figures that this is a mounting problem. The House of Commons is an old building, near a river with a huge basement area," he said.
Conservative MP Sir Paul Beresford, a member of the Commons Commission, told the Mail Online there is a "constant battle" against rodents in Parliament.
"All sorts of techniques are used to deal with this, right through to using a kestrel" Beresford added, although he said he was wasn't sure if the apparent "mutant-rats" were among the rodents seen in the Commons.
The British Pest Control Association's Richard Moseley said that this strain of rats have become so immune to regular poisons that they eat them "like feed".
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."