The world's "biggest ever" spider has been exposed as an impostor by a leading British spider scientist. 

Megarachne servinei was thought to be the most terrifying spider to roam the Earth and was in the Guinness Book of Records as the largest. 

Mystery has always surrounded the fossil of the pre-historic creature, locked away in a bank vault until now. 

But arachnid expert Dr Paul Selden, of Manchester University, said the metre-long beast was a sea scorpion. 

The fossil of the creature was discovered in 1980 by Argentine palaeontologist Mario Hunicken, who originally classified it as a spider, crawling the earth around 300 million years ago. 

Nightmarish models of the creature were exhibited around the world, but mystery always surrounded the findings which were not verified until recently. 

The original spider-like impression of the Megarachne servinei

Dr Selden was one of the first scientists allowed to examine Megarachne. 

He said: "As soon as I saw it I knew it wasn't a spider, but an ancient aquatic creature. 

"It has large claws and two big compound eyes, whereas spiders normally have eight small eyes.  It also appears to have a very robust body or shell with ridges across its back which is not found in any spider known to man. 

"This creature probably lived in a swamp and used its claws for sweeping up mud.  If you had to compare it to something which is alive today you would probably choose a large crab or a lobster, not a spider." 

Dr Selden's new classification of the creature has been accepted by Mr Hunicken, a spokesperson at the University of Manchester confirmed. 

Dr Selden said: "Even though this isn't the biggest ever spider, it clearly is an amazing beast."