Super six-inch Spanish cannibal slugs have now invaded 50 miles into Britain putting native British species of slugs, snails and beetles under threat

Britain is in the grip of an invasion from foreign cannibal slugs after the six inch-long monsters were spotted for the first time – fifty miles inland.

Giant Spanish Slugs are spreading across southern England destroying populations of our native slugs and snails by eating them.

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

They thrived following the mild winter of 2012 and have spread from the Kent coastline a whopping 50 miles inland to Tonbridge, eating our smaller, native species on the way.

The voracious predators will eat everything they can, including beetles, snails, other slugs, dog waste and leaves – as well as dead animals and valuable crops.

Dr Ian Bedford, an expert in the Spanish slug and a member of SlugWatch, 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.”

It is feared nothing can stop the foreign slime wave, as Spanish Slugs are notoriously tough to kill.

They can lay 400 eggs a year, more than twice our native breeds, and are light brown in colour, and have also been spotted in East Anglia and London.

Dubbed the worst slug pest in the world, a single killer slug, Arion vulgaris, can create an infestation all on its own as they can produce and fertilise their own eggs.

The only effective way to destroy the pest is either with boiling water, because they are too slimy to be eaten by birds or hedgehgs, and have to consume 20 slug pellets before they feel any adverse effect.

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

In 2012 Dr Bedford noticed an unusually large number of slugs outside his house in Norwich and collected more than 350 in 30 minutes – before coming to the conclusion he was being overrun with the invaders.

He said: “In my garden they wiped out all the vegetables I was growing and also a range of flowers and we’ve found them right down the east coast, as well as London and Kent.

“They’re further inland than ever before now, I think.”

Steven Falk, an entomologist with the charity Buglife, said our mild winter had boosted their numbers.

He 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.

“Most slugs can cope with winter. A lot of them survive in the egg stage under logs and things but the adults normally die out.”