For almost a quarter of a century, Steve Feltham has been the world's most devoted Nessie hunter.

But now the man who holds a place in the Guinness Book of Records for the longest continuous monster hunting vigil of Loch Ness has come up with a theory that Nessie is a giant catfish.

He said she is not a plesiosaur, as many believe, but was introduced for sport by the Victorians and only one remains.

If true, Mr Feltham's verdict would be a devastating to the £25 million a year Loch Ness tourism industry.

Mr Feltham, 52, who lives in a mobile home at Dores after quitting his home in Dorset in 1991, said she is a Wels catfish, which can grow up to 13ft in length and weigh 62 stone.

He said: "It is known they were introduced into English lakes by the Victorians for sport."

Their long life span would explain the sightings since the 1930s as the animals reached maturity.

Mr Feltham added: "I think the numbers have declined to the extent that there are now just one or two left. They also eat other catfish and may have eaten breeding females over time. Nessie is destined to be no more, I'm afraid."