A silver coin seen to represent a "key moment" in the development of an independent Scotland in the 12th century is expected to fetch £8000 at auction.

The penny, which bears the head of David I, the king of Scotland from 1124 to 1153, was recently found in a field in County Durham by a metal detectorist.

The piece is thought to have been created some time after the Scots invaded England in 1136 to intervene in a civil war over the English throne.

It will be auctioned off at international coin and jewellery specialist Dix Noonan Webb in London on September 22, when it is expected to fetch at least £8000.

Christopher Webb, head of the firm's coins department, described the item as "very important".

He said: "It is one of the first to be struck bearing the head of a Scottish monarch and so represents a key moment in the development of independent Scotland.

"It is a unique type never seen before and so is an exciting find in terms of the history of coinage."

The metal dectorist, who asked only to be named as Steve, found the coin in the field in May after discovering pennies from the reigns of the English kings Edward I and Edward III, a couple of Elizabethan coins, two modern gold rings and a large number of bullets.

The coin is believed to have lain in the soil for almost 900 years in an area of land which had been granted to David I under the Treaty of Durham in 1139.

It would have been struck some time after the Scots invaded England in 1136 to intervene on the side of Empress Matilda in her war against Stephen of Blois to decide who should succeed to the English throne.

The invaders captured Carlisle and its mint, where the first Scottish coins were created in the name of David I.

The field is due to be ploughed for the first time in living memory and experts believe the coin may not have survived that intrusion.

The metal detectorist said: "I am thrilled to bits, absolutely delighted. It is the best thing I have ever found and I started as a detectorist 42 years ago when I was just 12 years old."