Background

The coins of the United Kingdom from 1p to £1 were redesigned in 2008. The designs for the 5p and 10p had been unchanged for 40 years and there is an unwritten rule that designs should be changed at least every 40 years to keep the coinage fresh. The new designs usually coincide with the change of monarch, but as the Queen has reigned for so long, the change had to be planned.

However, this change to the coinage has resulted in ‘mule’ coins escaping into circulation. The term ‘mule’ is applied to a coin that has mismatched sides - a harsh and somewhat inaccurate take on the mule being a mismatch of a horse and donkey.

In this instance the mismatch involves the new design reverse side (tails) and the old design Queen's portrait side. The old obverse (heads), bearing no date, is married to the new reverse (tails). Ordinarily this would only result in some coins of 2008 being issued with an older date (say, 2007 for example). But with the creation of the new designs for the 2008 coinage the date on the 20p was moved to the Queen's portrait side. On the old coins the Queen's portrait side had no date, so the result of this mismatch is not an incorrectly dated 20p, it is a wholly undated one.

The last time an undated coin entered general circulation was over 300 years ago. This is testimony to the excellence of The Royal Mint, the facility that strikes all Britain's circulating coinage, which is at the forefront of minting technology the world over. Its stringent quality control procedures mean that error strikings such as the undated 20p are exceptionally rare indeed.

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