It is estimated at £15,000-20,000 and is being sold by the finder.

Jim Brown, coin specialist at Noonans, explained: “This is a find of considerable historical and numismatic importance.

“It is not really surprising that new varieties of medieval coins turn up from time to time - this is to be expected. What makes this find so different is the unique nature of the design - a complete and totally unexpected departure from the norm.”

He continued: “The original medieval fortress at Carlisle that was constructed in 1092, after the capture of the city from the Scots by William Rufus. Some 30 years later, Henry I decreed it be rebuilt in stone and ‘fortified with a castle and towers’. This is probably the medieval keep that can be seen today, albeit much altered in the interim.”

Mr Brown added: “The exact occasion of its striking however can only be surmised. It was a tumultuous period of alliances, battles and treaties changing with the ebb and flow of the political and military struggle.

“In view of the obverse type, it may be to commemorate David’s fortification of his new power base in Cumbria – an issue unparalleled in British medieval numismatics.”