It sounds like Cash in the Attic meets the The Da Vinci Code. A pile of junk cleared from a country home finds its way to a car boot sale in a nearby market town. Among the detritus is a small piece of wood measuring just 10 inches by four inches and covered with painted figures.

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Antiques dealer Martin Roberts suspected that the item being sold by a friend was worth a punt, and so he offered to swap it for a pine chest of drawers and six Victorian glass handles which he had bought for £13 and to hand back 10 per cent of the final sale price achieved.

The gamble now looks like it may pay off handsomely after the piece was identified as a possible tabernacle door belonging to the Knights Templar and dating back to the time of the Middle Ages.

Mr Roberts is hopeful that his latest find could match that of his most famous discovery – a 3,500-year-old Egyptian artefact he found in a box full of silverware during a house clearance near Harrogate.

Having paid £50 for that lot he eventually sold the four-inch royal shabti torso of Amenophis III, the grandfather of King Tutankhamun, at auction for £30,000 when a British-based private bidder paid 12 times the reserve price.

The former professional golf player astonished experts who insisted on wearing gloves to examine his latest find when he told them that he had driven around for two weeks with the tabernacle on the dashboard of his van.

"When I touched it, it sent shivers through me," he said yesterday. It was initially checked out by a dealer in Doncaster who suggested the polychrome cartoon images could be of St George and the Dragon. But he now believes it is more likely to be a Roman stabbing a Turk – a reference to the Crusades as well as a priest carrying a cross. A second expert suggested that its origins could be traced back to the Orthodox Church between 700 and 1200.

The door was found at Masham, North Yorkshire, close to Middleham Castle, the former home of Richard III which dates back to the time of the Norman Conquest. One theory is that it may have fallen into the possession of one of the influential residents that inhabited the castle, known as the Windsor of the North.

"At the end of the day I go around and find things. I buy from car boots and junk shops but I have never seen such a wonderful response as when people see this panel," said Mr Roberts, who would not be drawn on how much it was worth.

"We are trying to find any historians that might know about it. If it is significant I might want to put it on display in a museum. If it is part of our heritage it must not sit in some collector’s private display where no one can see it simply because he has got more carrots than anyone else," he said.

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Mr Roberts, who took up antiques dealing on eBay six years ago following the death of his wife from cancer, said he developed

an interest in the business through his father. "My dad sat me down and said to me, ‘Your brother is a roofer and your other brother is a steel erector – you are going to be an antiques dealer,’ and he gave me a set of books," he recalled.

"I don’t always buy plums. Sometimes I buy lemons and I have certainly bought plenty of them. It is a real buzz. The money is great but if you relied on the plums you would be starving," he admitted.

Mr Roberts, who has also discovered a bronze Egyptian figure dating back to 600BC, admitted that his knowledge of the history of Templar period was sketchy. "When it comes to the history I am on a Monty Python level. The best description of the Knights Templar is in Spamalot – I’ve seen that," he said.

"You can drop on stuff but you have to know what you are looking for. I have a wish list from customers who want me to find things for them." Mr Roberts added: "Maybe now we could sell the movie rights to this."