It sounds like the stuff of nightmares – giant earthworms that, if left alone, keep growing and growing to the size of a baby snake.

But this is no bad dream – scientists working on the Isle of Rum, off the coast of Scotland, have found the biggest specimens ever seen in the UK, more than three times the length and weight of a normal worm.

The exceptionally large invertebrates measure up 40 cm (1.3 ft) long, having blossomed due to rich soil and a lack of predators. They’re similar in size to a newly-hatched adder.

In an interview on the BBC Radio 4 Today programme, Dr Kevin Butt, lead researcher on the earthworm study, carried out by the University of Central Lancashire, said: “These things weigh about twelve and a half grams - but the normal size for these things is about four to five grams.”