The Spanish cannibal slugs have a voracious appetite and gardeners fear their prized crops and plants will be destroyed as they move inland

Britain is in the grip of an invasion from six-inch-long Spanish cannibal slugs - nicknamed killer slugs - that have been spotted 50 miles inland for the first time.

The Spanish slugs have a voracious appetite and gardeners and growers fear their crops and prize plants will be wiped out.

The gastropods - classed as among the 100 most invasive species in Europe - also feed on dead mice, meat and even each other.

They were first spotted in the UK in 2010 but cold weather helped kill them off.

But now they are spreading across southern England and have ventured 50 miles inland to Tonbridge, in Kent.

The Spanish slugs - which are officially called Arion vulgaris - have benefited from a relatively mild winter, which has allowed them to thrive.

The aggressive invaders cannot be stopped by normal methods because they are immune to poison.

They are also too slimy to be eaten by birds or hedgehogs.

Their aggressive breeding cycle mean they lay around 400 eggs in a year, compared to the 100 or so laid by native species.

There are fears they could breed with native species, creating a hybrid that can tolerate cold.

Another problem is the new diseases and parasites the killer slugs bring in to the country.

The gastropods - classed as among the 100 most invasive species in Europe - feed on dead mice, meat and even each other

Experts have warned of a disaster waiting to happen if they become established in the UK and urged people to kill them by pouring boiling water over them.

Dr Ian Bedford, the expert who first identified the species three years ago, said: 'If you find them in your garden you must kill them because they reproduce so rapidly.

'They produce so much slime that hedgehogs and slug-eating birds do not go near them, so they have no predators.

'An infestation can be devastating so if you find them in large numbers you must drown them overnight in a bucket of soapy water and then bury them in the ground.

'We have to keep down the numbers because they can decimate crops.'

The Spanish slugs cannot be stopped by normal methods because they are immune to poison and are too slimy to be eaten by birds or hedgehogs

SIX-INCH LONG SLUGS THAT EAT MICE, MEAT AND EVEN EACH OTHER Despite their name, Spanish slugs could have originated in western France. They eat dead or weaker members of their own species and other slugs. Like all slugs, they are hermaphrodites – meaning just one can produce an infestation. The Spanish slugs adapted to hot climates, where eggs are at risk of drying out, by laying hundreds. However, in our wet weather, most survive. Predators such as hedgehogs, and ducks control slugs – but this species is less likely to be tackled by these animals. A Cannibal slug is brownish in colour, is typically between eight and 12cm long and can be found in most of Europe. A typical black slug, in comparison, is normally between 10 and 12cm long and is mostly found in northern Europe. It is omnivorous, feeding mostly on dead animals, fungi and vegetation. Advertisement

The aggressive breed is believed to have come over to the UK on imported salad leaves and potted plants.

A large number were found in East Anglia 2013 and are believed to have hitched a lift on salad leaves imported from the Continent.

This time the mild weather and, in particular, the lack of morning frosts has allowed them to thrive.

The winter of 2012/13 saw several cold snaps, blizzards and frosts, which hit the new population of Spanish slugs hard.

But the intervening two winters have been much warmer, particularly in southern England.

The average temperature in England for December, January and February this year was 4.4C (39.9F), and the year before was even warmer at 5.8C (42.4F).

The average English winter temperature is 4.2C (39.6F).

Steven Falk, an entomologist with the charity Buglife, said: 'It was a very wet winter and a very mild winter - that plays into the hands of slugs. It was a very slug-friendly winter.

'Frost is the enemy of the adult and that normally what kills them off but we haven't had one recently.