A silver penny found by a metal detector depicting the Sir Alan Sugar of his day 900 years ago could fetch £15,000 at auction.

The 12th century treasure depicting Baron Eustace Fitzjohn, which measures just over half-an-inch across, was minted in York and is one of only 20 surviving examples of its design.

Rob Brown discovered the rare coin on a stubble field near Pickering, North Yorkshire, after walking just 20 paces to get his first positive metal signal of the day.

The 56-year-old from Leeds found the coin in August, just two inches down in a clump of soil.

On closer inspection he saw it had a design on one side of a standing helmeted knight holding a sword.

Around this is an inscription which reads 'EUSTACIUS' and on the reverse is a cross within a quatrefoil symmetrical design with the legend 'EBOR. ACIT. DEFT'.

Rob did not initially recognise the coin, which is a very rare silver penny issued in York by Eustace Fitzjohn, the Lord of Malton and Knaresborough, who served under King Henry I.

Eustace Fitzjohn became a wealthy landowner through marriage and later supported the Empress Matilda when she fought a civil war over who would inherit the English crown with her cousin Stephen in the period known as "the anarchy" in England.

There is a fortification which dates from the period of the conflict near where the coin was found.

Rob, who has been metal-detecting for eight years, said: "I was detecting with a friend and we found the coin, we weren’t sure if it was Saxon or Viking, so put a picture on a Facebook page devoted to milled and hammered coins – lots of people were reacting and were very excited – many knew instantly who or what it was."

In 1138, Eustace lost custody of Bamburgh Castle to Stephen and decided to join forces with David I of Scotland fighting with him against Stephen at the battle of the standard that year.

Then in July 1157, Eustace was killed in Flintshire after being ambushed by the Welsh army.

Nigel Mills, expert in artefacts and antiquities at auctioneers Dix Noonan Webb, described the baron as the "Alan Sugar of his day" because he came from a humble background.

He said: "Baron Eustace Fitzjohn was a Justician of the north and became a great monastic patron.

"He was to some extent, the Baron Alan Sugar of his day coming from a humble background but achieving great wealth and prominence.

"As it was a period of Civil War, Baron Fitzjohn had the authority to have coins struck in York which were primarily for local use."

The coin is estimated to fetch between £10,000 and £15,000 at auction.

Dix Noonan Webb will be holding a live online auction of Coins and Historical Medals on November 3 at 10am.