But they are probably not Jurassic Park-style survivors from the dinosaur age.

Scientists are to discuss the possibility of large undiscovered creatures in the sea at the Zoological Society of London (ZSL).

One of the speakers, science writer and palaeontologist Dr Darren Naish, said: "The huge number of 'sea monster' sightings now on record can't all be explained away as mistakes, sightings of known animals or hoaxes.

"At least some of the better ones, some of them made by trained naturalists and such, probably are descriptions of encounters with real, unknown animals.

"And, because new large marine animals continue to be discovered - various new whale and shark species have been named in recent years - the idea that such species might await discovery is, at the very least, plausible."

Some have suggested that present-day "monsters" might be plesiosaurs, long-necked marine reptiles that lived at the time of dinosaurs, or other survivors from the prehistoric world.

Dr Naish thinks this is unlikely and points out that the "prehistoric survivor paradigm" (PSP) contradicts what is known about the fossil record.

"The idea that these 'sea monsters' might be such things as living plesiosaurs is not a good explanation at all," said Dr Naish, who is affiliated with the University of Portsmouth.

The meeting, entitled "Cryptozoology: Science or Pseudoscience", is being organised by Dr Charles Paxton from the University of St Andrews.