A rare coin discovered by a metal detectorist who initially thought it was a button could fetch up to £12,000 at auction, say experts.

The Anglo-Saxon penny, which is more than 1,200-year-old, was found by a car salesman searching for artefacts and treasures in a farmer’s field in Sheldwich.

It bears the name of an early Archbishop of Canterbury and is only the second of its kind known to be in existence.

It was found by 53-year-old Ronnie Carlile during a metal detectorists’ rally last September and is now being auctioned by international coins specialists Dix Noonan Webb.

Mr Carlile, from Biggin Hill, said: “It was just a few inches below the surface and at first I thought it was a button because I had already found quite a few of those.

“It wasn’t until I took it back to the finds tent that I realised it was a coin and it was found to be Anglo-Saxon.”

However, the full importance of the coin did not become apparent until it was examined by experts at the Fitzwilliam Museum in Cambridge.

Mr Carlile said: “It was a shock to be honest. It is the rarest thing that I have ever found.”

The penny bears the name of Jænberht, who was Archbishop of Canterbury in the late 8th century.

It is only the second known coin of its time to have the name of the Archbishop on it instead Offa, the powerful king of Mercia, who was seeking to extend his control over much of the rest of England at the time.

Experts say the coin, which dates from 775-9, is an extremely rare relic of a brief period when Kent re-asserted its independence from Offa after defeating him at the Battle of Otford, near Sevenoaks in 776.

The auction is on June 14 in London and Mr Carlile will split the proceeds of the sale with the owner of the field.