They were once dismissed as the stuff of legend, alongside UFOs and the Loch Ness Monster.

But a surge in big cat sightings has convinced many experts that something is lurking out there.

Police are taking the big cat phenomenon seriously, even scrambling marksmen after a woman reported seeing a panther-like animal.

In Sussex the number of reports of pumas, panthers, lynxes and other big cats has soared, with police recording almost one case a month in the last two years.

According to experts, there can be only one explanation - they are breeding.

Figures released by Sussex Police revealed 20 sightings by members of the public during 2004 and 2005.

On February 14, 2004, a large cat was reported on the Downs by the Hove bypass on the A27.

The caller claimed the cat was brown, the size of a labrador, with a very long tail and spots.

Panthers have been spotted walking across a road in Petworth, sitting on a path behind houses on the Allards council estate at Guestling Wood, on an airfield at Gatwick, on the Cuckoo Trail at Hailsham heading towards the lambs and in Glebelands, Pulborough.

Firearms officers were deployed to Seaford in July 2004 after a woman spotted a big cat. The area was searched but the beast was never found.

The following month, an animal "the size of a leopard" with spots and a long tail was seen in Piddinghoe.

Attacks on sheep and horses in Sussex have also been blamed on big cats but no trace has ever been found.

Research from the British Big Cats Society reveals there were 2,123 sightings of big cats in the UK between April 2004 and July 2005.

Almost 60 per cent were of black cats and 32 per cent were of brown or sandy-coloured ones - believed to be pumas.

Another six per cent were of lynx-type cats.

Sophie Stafford, editor of BBC Wildlife magazine, which commissioned the study, said: "The thought of big cats roaming the British countryside has captured peoples imagination but until now, proving their existence has been impossible.

"With fresh new evidence and sightings on the increase, the society is now tantalisingly close to being able to provide conclusive proof."

It is widely believed that when the Dangerous Wild Animals Act 1976 came into force, many owners released their big cats into the wild rather than fork out on expensive cages.

But Guy Liebenberg, who works at Coastway Veterinary Group in Portslade, is not convinced.

The veterinary surgeon, who moved to England 15 years ago from Durban, South Africa, where he worked with wild animals, said: "It is highly unlikely there are big cats in Sussex.

"If there were, they would be hunting and we would notice remains of other animals.