Take a close look at that shiny new 20p piece in your purse or pocket - it could be worth a lot more than you think.

A rare error at the Royal Mint means that tens of thousands of the coins produced earlier this year don't have 2009 stamped on them.

Coin experts say the lack of a date makes them worth £50 each, and potentially much more in future. 

The blunder occurred after a redesign of the 20p piece. 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.

This is 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 error has caused a great deal of excitement among coin collectors because the Royal Mint, in Llantrisant, South Wales, is normally so careful about the manufacture and release of coins into circulation.