A unique silver penny dug up in a field near Nottingham last November has revealed the existence of a previously unknown mint during the reign of King Stephen (1135-54).

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The penny struck c.1140 at a previously unknown mint at Tutbury Castle. DNW have hopes of up to £10,000 for it in their April 2 sale.

The coin, of an unknown type, is expected to fetch up to £10,000 at Dix Noonan Webb on April 2. It was discovered by a metal-detecting enthusiast from Sheffield.

Examination has revealed that it was issued by Robert de Ferrers, second Earl of Derby, in the early 1140s during a period when royal control in England had all but broken down. At the time Stephen was fighting a bitter civil war, later known as The Anarchy, with his cousin Matilda over who should have the throne.

As the central authorities were not producing sufficient coinage, barons such as the Earl of Derby stepped in to provide currency.

The penny was struck at Tutbury Castle, near Burton upon Trent, the home of the Earl, by a moneyer called Walchelin, who was probably a member of his family. It is clearly related to a group of coins struck at Derby by the same man, DNW told ATG.

Not only is it unique but it was not previously known that Tutbury Castle, now largely ruined but still used for events, had been a mint.

"It still hasn't sunk in, it's the find of a lifetime," said the man who dug up the penny, who has been metal detecting for four years. "It's going to change the history of coinage at that time because the experts thought that everything that could be found had already been discovered."