A rare 14th century ring found in west Dorset by a local metal detectorist is estimated to fetch between £30,000 to £40,000 when it goes under the hammer.

David Board, 69, discovered the Lady Brook medieval diamond ring after gaining permission to search near Thorncombe by a farmer, for whom David had formerly delivered milk to for many years in his previous profession.

Having detected on local beaches in the 1970s and not finding much, a family friend motivated David to try his luck again recently.

Armed with the latest in metal-detecting equipment, he spent two outings on a pasture field.

Nearing the end of the day, and having found just a few old copper half pennies, he got a signal near a footpath.

At a depth of five inches, he saw what he thought was a sweet wrapper, then upon looking more closely he realised it was a ring and put it in his top pocket.

Finds Liaison Officer for the region Lucy Shipley took the ring to the British Museum and was able to confirm that it was medieval in date and a very rare example.

The ring is suggested to be the wedding ring given by Sir Thomas Brook to his wife Lady Joan Brook for their marriage in 1388.

On Tuesday, November 29 the ring will be offered by Mayfair auctioneers Noonans in its auction of jewellery and watches.

It is expected the proceeds will be split between the landowner and the finder.

David is hoping to use his share of the money to help his partner's daughter arrange a mortgage.

Nigel Mills, consultant (coins and antiquities) at Noonans said: "This ring is almost perfect condition and has an inverted diamond set into the raised bezel so that it comes to a point.

"The hoop is composed of two neatly entwined bands symbolising the union of the couple.

"Inside the band is an inscription in French 'ieo vos tien foi tenes le moy' (translating as 'I hold your faith, hold mine')".

Another metal-detecting find was due to be sold at auction this week.

Andrew Phillips, 69, of Hertfordshire, discovered a silver seal from the 14th century which was expected to sell for £3,000-£4,000 at auction.