Rare Royal Mint 20p coins have been selling for a small fortune on online auction site, with three coins selling for more than £150 this month.

The 20p pieces are “mule” coins dating from 2008 and were minted without a date on the coin due to an error.

The mistake means the coins are considered collector’s item.

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

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.

Chester and District Standard: The three coins sold for £151.99 on eBay, after attracting 34 bidsThe three coins sold for £151.99 on eBay, after attracting 34 bids (Image: eBay)

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.

Three of those coins going under the hammer at the same time was too good an opportunity to waste for some collectors, as a bidding war ensued.

Eventually the three coins sold for £151.99 on eBay, after attracting 34 bids.

Rarest Royal Mint coins in circulation the UK?

The rarest is the Kew’s Garden 50p, which was designed to mark the 250th anniversary of the gardens in 2009. Only 210,000 coins were ever minted with this design.

The Kew Gardens 50p sells for £156.25 on average, but one seller received over £700 for one when they sold it on eBay.

The other rarest coins stem predominantly from the 2011 Olympics, with the wrestling, football and judo coins among the most valuable. Only 1.1million of each of these coins were produced.

Flopsy bunny and Peter Rabbit designs which were produced in 2018 are also highly valuable.

These coins - 1.4 million of which were minted - depict the characters from Beatrix Potter’s novels and celebrate the life of the English writer and these sell for around £5.

In 2019, 500 million coins were produced, with three new 50p designs.

These included one of Arthur Conan Doyle’s Sherlock Holmes, Paddington Bear at St Paul's Cathedral and the Tower of London.