A terrifying giant scorpion measuring 3.2 feet (one metre) long roamed Britain 415 million years ago, a new study has found.

With a length comparable to that of a Labrador and armed with pincers over 6.3 inches (16cm) long, Praearcturus gigas would have been a formidable apex predator.

It stalked the floodplains of what is now England and Wales long before trees even evolved.

Scientists from the Natural History Museum have now declared the scorpion the largest of its kind ever to exist.

And its anatomy suggests it was capable of moving easily between water and land.

'When we think of giant arthropods, people often picture Carboniferous rainforests with giant millipedes or dragonfly-like insects from later in Earth's history,' Dr Richard J. Howard, Curator of Fossil Arthropods at the Natural History Museum, said.

'But Praearcturus lived at least 50 million years earlier, well before the evolution of trees, when life on land was only just getting started.

'Confirming that this animal is a scorpion fundamentally changes our understanding of how and when these creatures evolved to such extraordinary sizes.'

The fossils that have been used to identify Praearcturus have actually been held in the Museum's collection for more than 150 years.

Scientists used modern analytical techniques and comparisons with newly described fossil species to confirm that Praearcturus is definitively a scorpion and a distinct species.

It lived during the Early Devonian – around 415 million years ago when life on land was still in its infancy.

Small plants and fungi had only recently begun to spread across the landscape, and complex terrestrial ecosystems like forests had yet to evolve.

This means that, unlike later giant arthropods, Praearcturus did not benefit from the high atmospheric oxygen levels associated with the rise of forests.

Instead, its enormous size may reflect a world with relatively little competition from other large predators.

This suggests the scorpion might have grown so big simply because there weren't many other large animals around, meaning it could dominate its environment in a way that wouldn't be possible later on, experts said.

They also revealed the predator had flap-like structures on its abdomen, similar to those found in modern crustaceans such as lobsters. This places Praearcturus at a pivotal moment in Earth's history when animals were first experimenting with life outside the oceans.

Dr Greg Edgecombe, Merit Researcher at the Natural History Museum and co-author of the study said: 'The boundary between land and sea was much less defined at this time.

'Praearcturus gives us a fascinating glimpse into how early animals adapted to these changing environments.

'It may even represent a lineage that returned to the water after earlier ancestors had already begun living on land.'

First described in 1871, Praearcturus gigas was originally thought to be a giant crustacean, similar to a woodlouse.

However, the known fossils lacked key features such as a tail, making it difficult to classify with confidence for more than a century.

The breakthrough came through comparison with better preserved fossils discovered in recent years, which revealed key anatomical features unique to scorpions.

Dr Howard added: 'Specimens collected over a century ago can still hold entirely new insights. By revisiting them with modern techniques, we can uncover discoveries that reshape our understanding of life on Earth.'

The discovery of such a large scorpion so early in the history of life on land challenges assumptions about why prehistoric arthropods reached gigantic sizes. Rather than being driven solely by environmental factors such as oxygen levels, the findings suggest that ecological opportunity such as a lack of competition may have played a crucial role.

So far, fossil evidence of the giant scorpion has been found in Rowlestone and Longtown in Herefordshire and in Trimpley in Worcestershire.

Fossils have also been uncovered in Tredomen Quarry near Brecon in Wales.

'We suggest that Praearcturus was an apex predator and may have been at least partially aquatic,' the researchers wrote in the journal Palaeontology.

While its size would have made it a terrifying creature to come across, experts have previously revealed that smaller species of scorpions have more potent venoms.

Researchers from NUI Galway analysed 36 species of scorpions and found that the smallest species were 100 times more potent that the largest.