Why do thousands of us think we've seen big cats every year?

The disc jockey from South London was quite clear about it. A big black cat seized him in his front garden on Tuesday. Five and a half foot tall on its hind legs, it was. And ferocious with it. Tony Holder's face bears the scratches, his hand the mark of its teeth; and now, he says three days later, the foxes at the bottom of the garden have disappeared.

The disc jockey from South London was quite clear about it. A big black cat seized him in his front garden on Tuesday. Five and a half foot tall on its hind legs, it was. And ferocious with it. Tony Holder's face bears the scratches, his hand the mark of its teeth; and now, he says three days later, the foxes at the bottom of the garden have disappeared.

The man from the British Big Cat Society (BBCS) was less convinced. He hightailed it to the scene, examined the victim, and declared the marks he bore were probably not inflicted by a cat. The local wildlife warden was also a sceptic. "Well, we've got foxes here, and woodmice," he said, "But no big paw prints."

And so, for now, the Mystery of the Sydenham Puma can be added to the Cricklewood Lynx and the Beasts of Exmoor, Bodmin, Barford and elsewhere. In 2003, nearly 2,000 claimed to have seen a big cat in Britain, double the number the year before.

Can the eyes of so many people deceive them? Or is there something out there? The Independent on Sunday set out to follow the indistinct tracks. They led, if anywhere, straight up the garden path.

The BBCS says between 60 and 100 big cats roam Britain. Its evidence includes the sightings, carcasses found in the last 30 years (two lynx, six leopard cats, similar in size to a tall domestic cat, and four jungle cats, a breed one-third larger than a domestic), some fuzzy photographs, paw prints, and livestock kills. But no clear pictures of a feral big cat have been taken, expert trackers have failed to confirm a single paw print as that of a big cat, and DNA evidence from the remains of livestock allegedly killed by big cats has suggested dogs or badgers. And few, if any, of the kills exhibited the classic go-for-the-throat signs of a big cat attack - many bear the marks of dogs, either ferals or the lurchers used by some lampers and livestock poachers.

The sightings, many sincere but few from those with wildlife expertise, come from all over the UK. In some cases, remote cameras with sensors have been set up but have never captured any big cat image. The Department for the Environment, Food and Rural Affairs (Defra), has sent investigators to examine scenes of livestock kills, but found no evidence of big cats. Sarah Christie, carnivore programme manager at the Zoological Society of London, said: "A few years ago, we had a report of a lion on the loose in north London. We sent a crew up there with nets and so on. It turned out to be a ginger tom."

So whence are these big cats supposed to come? The traditional explanation is from exotic pet owners who have released them or allowed them to escape in the wake of the Dangerous Wild Animals Act 1976. But that was nearly 30 years ago, and the average life span of a big cat is about 15 years. Nor does the RSPCA "think there are big cats out there breeding".

Customs and Excise report no big cat seizures in the past four years, indicating few entering the country. The RSPCA says that cases of owners asking them to deal with exotic big cats are "extremely rare", and knows of only one recent case of big cats being rescued - two lynxes from a shed in Yorkshire in 2001. As for escapes from mainstream zoos, Ross Smitt of the British and Irish Association of Zoos and Aquaria said: "I can't think of any cases of big cats escaping off site in the past 20 years."

Ms Christie does concede that a big cat might come from "one of our less reputable small zoos or wildlife parks". The RSPCA agrees that short-term survival of a big cat is conceivable, as is the possibility of feral cats (and an unneutered tom is larger than the domestic cat) producing black off-spring. And, in Scotland, such animals could be a Kellas, a black cross between a wild and domestic cat.

Ms Christie "remains highly sceptical - 99 per cent, if not all, of these reports are rubbish. The whole issue is swamped by hysteria." As she says, after decades of sightings, "You would think there would be one fecal sample, one set of prints, but we have none of these things." Meanwhile, the believers go on believing and go on looking...