New 20p = £50

[Sikhishop.com]

Around 200,000 of the coins have rocketed in value after the manufacturer admitted it had forgotten to put a date on them.

They are worth £50 if immediately returned, but are set to rise even further in value because of their rarity.

Nick Hart, of the London Mint, said: "If you hunt around and find one you're extremely lucky.

"Mistakes like this not only fascinate people but also have considerable value-increase significance."

Workers at the mint have been left with their tails between their legs after leaving the date stamp off the heads side of the coins.

The error occurred amid the production of a newly-designed 20p piece.

The mint has issued a recall both to remove the coins from circulation and to keep track of them.

Lucky owners are asked to register their details at www.undated20p.com and then post the dateless coin back.

Alternatively, those looking to keep their new-found collectable can register and request a presentation box.

Mr Hart put an optimistic slant on the mint's "extremely rare error".

"(It) will certainly get the public looking at the coins in their pockets again and noticing the excellent new designs launched by the Royal Mint on our coinage," he said.