According to biologists, the slug has never been seen before in Western Europe, biologists say, and they are amazed that it has turned up in a garden in Caerphilly, north Wales.

Bill Symondson, an ecologist at Cardiff University, said: 'The lack of eyes and body colour could indicate the species evolved in a cave system.

'It was probably introduced to Britain in plant pots, making it an 'alien' species, although we can't be certain. We're concerned that it might become a pest, but we need to find out more about it first.'

Gardeners and garden centres are being urged to to inspect plants and pots to stop the slugs escaping and breeding.

There are fears it could hit the eco-system because it feeds on earthworms, which are vital for the well-being of soil.

A spokeswoman for the National Museum in Cardiff said it's a mystery how the slug got to Britain.

She added "Unlike most slugs, the ghost slug is carnivorous and kills earthworms at night with powerful, blade-like teeth, sucking them in like spaghetti.

'It has no eyes, is completely white, and lives underground, squeezing its flexible body into cracks to get at the worms.'

Ben Rowson, a biologist at the National Museum in Cardiff, said ' 'We had to thumb through lots of old publications in Russian and German to find anything like them" but then discovered they were something entirely new.They may well eat other slugs too.'

After examining the ghost slug biologists discovered that it is a completely new species.

They decided to name the creature Selenochlamys ysbryda, partly from the Welsh word ysbryd meaning ghost.