Scientists knew from fossils that the pterosaur, which roamed land and seas all over the world for hundreds of millions of years, had a wingspan of up to ten metres. Now analysis of fossilised footprints and bone fragments has confirmed some specimens were almost twice as big, with wingspans of 18 metres, making them as big as a medium-sized commercial aircraft.

The discovery was announced by fossil specialists yesterday at the British Association for the Advancement of Science's annual meeting in Dublin.

"These finds are quite amazing - we already knew this was the largest animal ever to fly but no one thought they would be this big," said David Martill, of Portsmouth University.

"These are truly enormous creatures and we keep having to revise our estimates upwards - we're starting to see the same thing as happened with fossils of the big land-based dinosaurs where we kept having to increase our estimates upwards from 50 tonnes to 60 tonnes, then 100 tonnes."

Evidence that pterosaurs were much larger than previously thought comes from fossils of a sub-species called azdarchids found in Israel and Mexico.

"The azdarchid fossils are small fragments but small fragments of something very big," said Dr Martill. "We now know the wing bones of their little finger were 12cm across which doesn't sound very much but that is phenomenal when you convert it into wingspan.

"One of the reasons why they grew so large may be that, unlike birds, they just carried on growing. If you have an individual who keeps on growing until he is about 75 or 80 he's going to be very large indeed."

Using computer three-dimensional imaging, scientists have calculated that the Israeli pterosaurs had a 14m wing span. But a new discovery of fossilised footprints in Mexico indicates some individuals had a wingspan of more than 18m.

Pterosaurs were fish-eating reptiles which lived at the same time as the dinosaurs from the Triassic period 220 million years ago until the Cretaceous period about 65 million years ago. Their nearest living relative is the crocodile.

They had huge elongated heads and jaws armed with razor-sharp teeth which they used to gobble up fish or animals while swooping low over water or land.

Their enormous sail-like wings, made of a thin hair-covered membrane between their two front limbs, allowed them to use air currents to fly with little effort over huge distances.

"Their flight membranes could be controlled by adjusting the angle of the forelimbs at the shoulder, the elbow and at the base of the hyper-elongated flight finger," said Dr Martill. "In addition it could be controlled by movement of the hind limbs at the hips, the knees and to some degree at the ankle.

"This gave pterosaurs far more flight control than birds of equivalent size."

Analysis of fossils has shown the intricate details of its super-strong but light bone structure. Dr Martill said they could even be used to help with modern aircraft design.

"They took bone to new limits in terms of making it thin yet strong," he said. "Their skeletons were very lightly constructed and most of their bones were hollow and enclosed an air sac system connected to the lungs.

"The bone itself was composed of many microscopically thin layers stacked together like a spirally bound plywood tube.

"Sometimes the bones had cross-sectional shapes that provided added strength, such as D, T and A shapes."

It appears pterosaurs could fly from the moment they hatched.

"Fossils of pterosaur eggs are very rare, but using computer imaging we've been able to see that the wing bone elements were all there so theoretically it was ready to fly as soon as it hatched. It's possible the females just left the eggs on a pile of seaweed and left them to fend for themselves."

How they mated is a matter of speculation but Dr Martill added: "I suspect they did it with gusto. The size of their head crests made of both bone and soft tissue suggests these crests may have played a role in sexual attraction therefore I suggest they may have indulged in flamboyant sexual activity."