Giant rats
A hungry rat

Pest controllers are reporting an increase in sightings of large rats in the UK. In Swindon, Justin Holloway of

In Swindon, Justin Holloway of local company Prokill recently discovered a 20-inch (50cm) rat.

Last year a rat was killed in Liverpool which measured no less than 2 feet (61cm) nose to tail. It was caught on an industrial estate by an operative from a local pest controller, Whelan Services.

But even this was not an isolated incident. A municipal pest controller in Birmingham, Colin Watts, has also reported seeing a rat of similar size.

Sightings of rats of all sizes are on the rise. As well as Liverpool, Whelan services has offices in London and Hampshire. It reports increased sightings of rodents in all its locations.

Liverpool Council has also experienced significant rises in rat sightings; according to the latest available figures there were 2,008 in 2013, against 1,860 the year before. Nationally, hard data for increases is limited. Estimates of the UK's rat population – seen and unseen – range between 10-80 million. But anecdotal evidence suggests that rats in general, and big rats in particular, are on the up.

Back in Swindon, Prokill's Holloway says that the increasing volumes of food waste are behind the appearance of the larger rats.

"If there's a good source of food and water it becomes a larger rat," he told the Daily Mirror.

Other factors include the migration of vermin from the countryside to cities. And cutbacks by local councils in pest control budgets following the advent of the financial crisis began have also been cited as contributing to the rise in visibility of bigger rats.

However, reports of the evolution of "super rats" are misleading. It has always been possible for rats to get bigger if generous food supplies are available and sightings of rats as big as cats go back a very long way.

Nevertheless, rats are becoming resistant to existing pesticides, which means that their chances of surviving and growing are better, particularly if humans are careless in disposing of food waste.

"The implication is if you are a rat and you're not being taken on, you might enjoy a longer life and grow to a larger size, because you are not being taken down, " said Prokill's Holloway.

From the '60s onwards pest controllers used warfarin (used by humans as a blood-thinning agent). When it became less effective bromadiolone, another anti-coagulant, was substituted in the '80s.

But both are proving ineffective of many of today's rats and pest controllers are calling for new poisons to be developed and adopted.