A metal detectorist has been left stunned after he discovered a Roman coin he unearthed 30 years ago was the only one like it in the world and worth £10,000.

Tom Thomas, 62, from Reading, said he thought nothing of it when he dug up the silver coin several decades ago in a farmer's field in Berkshire.

The retired police officer put in his small collection of other artefacts and forgot about it until a chance meeting with a fellow detectorist at a barbecue.

Mr Thomas began chatting to Mark Becher at the family gathering two years ago who thought the find was more unique than it was first thought.

The pair began contacting experts and Mr Thomas got the surprise of his life when it turned out to be a 2,000-year-old Carausius Denarius Roman coin.

The historic item, which dates back to AD 286-93, will now go under the hammer at Hansons Auctioneers with a guide price of £10,000.

The artefact, which is registered with the Portable Antiquities Scheme (PAS), features Roman goddess Salus feeding a snake rising from an altar.

Mr Thomas said: "I remember the day I found it.

"I was getting a very low signal from my metal detector. I wasn't going to dig but changed my mind.

"I had to dig down really deep compared to other coins, about eight inches, to reach it.

"I didn't know what it was as such.

"I knew it to be a Roman coin as I had found others in the past. I put it with my small collection and thought nothing more of it.

"I've been metal detecting for more than 30 years and I've found lots of different coins and other Roman artefacts.

"It turned out to be the only one of its kind in the world.

"But for that family barbecue and Mark spotting it, I may never have known.

"Mark contacted other experts, including the British Museum, and no-one had seen anything else like it before.

"I was surprised and delighted when I heard how special the coin was.

"The only reason I'm selling it now is because it's so unique and valuable it has to be locked away in a bank vault."

Mr Becher, who runs the Metal Detectives Group, organises digs and works as a consultant for Hansons, said: "I was staggered when I saw the coin.

"I've been metal detecting for more than 25 years and I've witnessed countless finds, both my own and other people's. I'd just never seen anything like it.

"After noticing it in Tom's collection, I quickly sent a picture of it to a good friend and Roman coin guru Chip Gruszczinski.

"He came back in a flash confirming what I'd assumed.

"I then contacted an absolute expert in the field of Roman coins, Sam Moorhead at the British Museum.

"He agreed with the consensus of opinion. People talk about rare but this coin is unique.

"As far as we know, it's the only one of its kind in the world. For keen collectors, it doesn't get any better than that."

The coin's PAS description adds: "The coin is not published and is the only example of its type in Sam Moorhead's [The British Museum's finds advisor for Roman coins] corpus for a new edition of RIC [Roman Imperial Coinage catalogue]."

It will be sold in Etwall, Derbys., on August 27.