The 150 million-year-old specimen was found on Spitspergen, in the Arctic island chain of Svalbard, in 2006.The Jurassic-era leviathan is one of 40 sea reptiles from a fossil "treasure trove" uncovered on the island.
Nicknamed "The Monster", the immense creature would have measured 15m (50ft) from nose to tail.And during the last field expedition, scientists discovered the remains of another so-called pliosaur which is thought to belong to the same species as The Monster - and may have been just as colossal.
The expedition's director Dr Jorn Hurum, from the University of Oslo Natural History Museum, said the Svalbard specimen is 20% larger than the previous biggest marine reptile - another massive pliosaur from Australia called Kronosaurus.
"We have carried out a search of the literature, so we now know that we have the biggest [pliosaur]. It's not just arm-waving anymore," Dr Hurum told the BBC News website.
"The flipper is 3m long with very few parts missing. On Monday, we assembled all the bones in our basement and we amazed ourselves - we had never seen it together before."Pliosaurs were a short-necked form of plesiosaur, a group of extinct reptiles that lived in the world's oceans during the age of the dinosaurs.
A pliosaur's body was tear drop-shaped with two sets of powerful flippers which it used to propel itself through the water."These animals were awesomely powerful predators," said plesiosaur palaeontologist Richard Forrest.
"If you compare the skull of a large pliosaur to a crocodile, it is very clear it is much better built for biting... by comparison with a crocodile, you have something like three or four times the cross-sectional space for muscles. So you have much bigger, more powerful muscles and huge, robust jaws.
"A large pliosaur was big enough to pick up a small car in its jaws and bite it in half.""There are a few isolated bones of huge pliosaurs already known but this is the first find of a significant portion of a whole skeleton of such a giant," said Angela Milner, associate keeper of palaeontology at London's Natural History Museum
"It will undoubtedly add much to our knowledge of these top marine predators. Pliosaurs were reptiles and they were almost certainly not warm-blooded so this discovery is also a good demonstration of plate tectonics and ancient climates.
"One hundred and fifty million years ago, Svalbard was not so near the North Pole, there was no ice cap and the climate was much warmer than it is today."
The Monster was excavated in August 2007 and taken to the Natural History Museum in Oslo. Team members had to remove hundreds of tonnes of rock by hand in high winds, fog, rain, freezing temperatures and with the constant threat of attack by polar bears.
They recovered the animal's snout, some teeth, much of the neck and back, the shoulder girdle and a nearly complete flipper.Unfortunately, there was a small river running through where the head lay, so much of the skull had been washed away.A preliminary analysis of the bones suggests this beast belongs to a previously unknown species.
The researchers plan to return to Svalbard later this year to excavate the new pliosaur.A few skull pieces, broken teeth and vertebrae from this second large specimen are already exposed and plenty more may be waiting to be excavated.
"It's a large one, and has the same bone structure as the previous one we found," said Espen Knutsen, from Oslo's Natural History Museum, who is studying the fossils.
Dr Hurum and his colleagues have now identified a total of 40 marine reptiles from Svalbard. The haul includes many long-necked plesiosaurs and ichthyosaurs in addition to the two pliosaurs.
Long-necked plesiosaurs are said to fit descriptions of Scotland's mythical Loch Ness monster. Ichthyosaurs bore a passing resemblance to modern dolphins, but they used an upright tail fin to propel themselves through the water.
Richard Forrest commented: "Here in Svalbard you have 40 specimens just lying around, which is like nothing we know."Even in classic fossil exposures such as you have in Dorset [in England], there are cliffs eroding over many years and every so often something pops up. But we haven't had 40 plesiosaurs from Dorset in 200 years."
The fossils were found in a fine-grained sedimentary rock called black shale. When the animals died, they sank to the bottom of a cold, shallow Jurassic sea and were covered over by mud. The oxygen-free, alkaline chemistry of the mud may explain the fossils' remarkable preservation, said Dr Hurum.
The discovery of another large pliosaur was announced in 2002. Known as the "Monster of Aramberri" after the site in north-eastern Mexico where it was dug up, the creature could be just as big as the Svalbard specimen, according to the team that found it.