An 18th-century gold coin worth up to £250,000 has been discovered in a child's pirate treasure collection.

The rare Queen Anne Vigo five guinea piece was given to the anonymous owner by his grandfather when he was a child.

The coin was kept in a toy box for pirate games and the owner had no idea of its value.

It was packed away and forgotten about until his grandad died when he rediscovered the coin and gave it to his own son to play with.

Experts then told him it was one of 20 made of gold seized from Spanish treasure ships in Vigo Bay, Spain, in 1702.

The coins, made out of treasure captured by the British fleet, were struck in 1703 as part of a propaganda campaign to detract attention from the British failure at Cadiz the previous year.

The treasure was delivered through London and received at the Royal Mint by then Master of the Mint, Sir Isaac Newton.

The man, of Bishop's Stortford, Essex, eventually took it to be valued.

Gregory Tong of Boningtons auctioneers, in Chelmsford, instantly recognised its value as one of fewer than 15 known examples.

It is expected to fetch up to £250,000 at auction next month and is only the sixth example of its type to be offered for sale in the last 50 years.