Sussex sculptor David Cornell's smiling likeness of Prince William will be used on new £1,000 coins, struck to coincide with his coming of age.

The Royal Mint will produce 21 of the Prince William Alderney gold kilo coins in 22-carat gold - each priced £14,500 - and 500 silver kilo coins with a face value of £50, price £695.

The £1,000 coin is the highest denomination ever seen in UK currency.

Prince William's face will be on one side. The other will bear the familiar Raphael Maklouf portrait of the Queen, which was used on all coins until 1998.

Mr Cornell, 67, from Crowborough, was determined to create something slightly different when commissioned to make Prince William worth his weight in gold - especially after receiving royal instructions.

He has said: "Coins traditionally feature a profile and not someone smiling. But Prince William insisted that was what he wanted."

Mr Cornell spent a month studying photographs taken by George Bodnar showing different angles of William's face.

He was an engraver with the Royal Mint during the Sixties and has also produced portraits of Diana, Princess of Wales and the Queen Mother.

He has met the Queen and dined with the Duke of Edinburgh, proving his value to the Royal Family across the generations.

His latest designs also feature on the newly-issued Prince William Alderney £5 crown.

This is available in a £9.95 presentation folder, with the coin also bearing the words HRH Prince William of Wales and the young royal's own coat of arms.

A silver three-coin set incorporating £5 crowns from Jersey, Guernsey and Alderney has been minted, with 5,000 copies available at £99 each.

A similar three-coin set in gold, selling for £1,745, has already sold out.

The coins from Jersey and Guernsey feature Mr Cornell's two further portraits of William, backed by Ian Rank-Broadley's portrait of the Queen, which has been used on all UK coins since 1998.

Gerald Sheehan, chief executive of the Royal Mint, said: "All of the coins which have been minted to celebrate the 21st birthday of Prince William are expected to be popular with collectors and members of the public alike."

The commemorative coins will be available from today until September 30.

To reserve coins, call the Royal Mint Coin Club on 0845 608 8300 or visit www.royalmint.com