Metal detector club chairman Jack Tree struck lucky while taking part in a charity rally at Winterbourne Bassett.

He did not find any of the tokens planted by the Round Table organisers, which would have won him a small prize.

Instead he made the find of the day, and possibly of his lifetime, when he unearthed a medieval gilded silver brooch.

Amazingly, after being in the ground for possibly 600 years or more, the brooch was in good condition and still had its pin.

When he found the brooch in the ploughed field it was encrusted with dirt.

Mr Tree told Wiltshire Coroner David Masters at a treasure inquest yesterday: "You cannot believe the feeling of picking up something like that.

"It was complete with its pin, most of the time they are not still there.

"How it had survived the field being ploughed thousands of times I don't know."

To be declared treasure an object has to be more than 300 years old and contain no less than ten per cent of precious metal.

Mr Tree said he was unsure of the age of the brooch and believed from his experience that it was made of bronze, not a special metal.

However when he took it home to Milford Haven - where he is chairman of the local metal detector club - and cleaned it up he realised it was not bronze.

Mr Tree said: "When I got it home and took the muck off, instead of there being a green patina under the gold, which you would find with bronze, it was grey which probably meant it was silver."

He immediately reported his discovery to Katie Hinds, the Wiltshire finds recording officer at Salisbury Museum.

The brooch was sent to the British Museum where it was confirmed as being a medieval silver gilt annular brooch with a design including ten moulded knobs and a pattern that appeared to be a Lombardic cross.

Mr Masters said the British Museum had dated the brooch, which the Wiltshire Heritage Museum in Devizes hopes to acquire, as 13th century. It had not, however, been valued yet.

He declared the brooch treasure and told Mr Tree it would be valued by the British Museum and that the finder and landowner David White would split whatever it sold for 50-50.