A man who has spent 24 years scanning Scotland's Loch Ness for its legendary mysterious monster reckons Nessie is most likely a giant catfish -although he is not prepared to give up looking just yet.

Steve Feltham, who holds a Guinness World Record for the longest continuous Nessie vigil, says it is the most probable explanation for the enigmatic beast that has captivated people's imaginations the world over.

"The current frontrunner is the Wels catfish. It's the most likely explanation," the 52-year-old told AFP.

"I'm not saying it's the final explanation. It ticks most of the boxes with sightings - but it doesn't tick them all."

Feltham left his home and girlfriend in 1991 to go looking for Nessie and has spent the years since in a caravan on the lake shore, scanning the waters.

Media reports this week suggested Feltham had given up his vigil after favoring the catfish theory, but he insisted he would keep searching for the definitive conclusion.