A rare error at the Royal Mint means tens of thousands of 20p coins produced earlier this year are worth £50 each.

The coins do not have the date stamped on them because of the blunder after the redesign of the 20p piece, reports the Daily Mail.

The Mint does not know exactly how many undated coins were produced and released into circulation, but estimates range between 50,000 and 200,000.

It's the first undated British coin to enter circulation in more than 300 years - the last occasion was 1672, when Charles II was on the throne.

The problem occurred after all the coins from the 1p through to the £1 were redesigned last year. The change of the 20p design meant moving the year date from the tails side to the side with the Queen's head.

However, thousands of coins were minted using the old version of the Queen's head, which does not have the year date, and the new version of the tails side, which also does not have a date.