We are all being encouraged to check our change after a rare 20p coin sold for 390 times its face value this week.

The 20p piece is a “mule” coin dating from 2008 and was minted without a date on the coin due to an error.

The mistake means the coin is considered a collector’s item.

A batch of coins was issued without dates in 2008 due to an error in the minting process.

Gazette: An example of a rare undated 20p mule coinAn example of a rare undated 20p mule coin (Image: eBay/Getty/Vandervelden)

Prior to November of that year, the date on 20p coins would be found on the tail side. However, the Royal Mint switched it so that the date would now be next to the Queen’s head.

A mistake in 2008 led to the old format being used on the Queen’s side and the new format being used on tails side.

As a result, the first batch of British coins with no date on them was released into circulation for the first time in more than 300 years.

Around 250,000 20p coins minted in November 2008 are believed to have entered circulation with no date.

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And now one has gone up for sale on eBay, attracting a winning bid of £78, 390 times its face value.

The description said: “This rare 20p coin from the Royal Mint boasts a unique error that makes it a must-have for any serious collector.

"Minted in 2008, this undated mule coin features the iconic image of the Queen's head on one side and the denomination on the other. With no date, this coin is a true rarity that is sure to impress.

"The coin was minted in Great Britain and is a part of the decimal coinage series that began in 1971. It is a perfect addition to any collection of British coins, and its error makes it a valuable piece for collectors who are looking for unique items. Don't miss your chance to own this rare and fascinating coin."

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What makes a coin valuable?

The 50 pence piece has become the most valued and collected coin in the UK, with many collectable designs appearing on its heptagonal canvas.

Its 27.5mm diameter makes it the largest of any British coin, and allows space for decorative pictures. It has often been used to celebrate big events over the past 50 years of British history.

The rarest coins tend to be of the greatest value, with the mintage (number of coins with each design made) being the fundamental attraction for collectors.

Along with the design, other aspects of the coin which increase value are the condition of the coin and whether it has an error in its design.

The way in which it is sold can also determine the coin’s value - while some coin collectors will bid vast amounts of money on ebay or at auction, others opt for more robust valuations by selling via a coin dealer.