Do you have one of these valuable £2 coins?

It's time to check your purse and look down the back of the sofa again: the Royal Mint has confirmed that a small number of £2 coins have a mistake that means they could be worth a fortune.

First discovered by the Change Checker website, the error in striking the Britannia £2 coins means that the Queen's head is rotated by around 150 degrees from its correct position.

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Even the correct version of the coin is comparatively rare: only 650,000 are in circulation, making it one of the three rarest circulating £2 coins. This means that the so-called Inverted Effigy version is all the rarer.

"The Royal Mint is unable to give any indication of how many Inverted Effigy coins have entered circulation but we can make some initial estimates," says Change Checker's Yasmin Britton.

"We have analysed 5,000 circulation coins and our results suggest that the Inverted Effigy may have affected as few as 1 in 200 of the coins struck – in other words around just 3,250 coins."

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The Royal Mint has confirmed the mistake, explaining that it's probably caused by the die having slipped during the striking process. And, says Britton, it's possible that other coins may exist on which the Queen's head is out of position by a different amount.

As yet, it's not known how much the coins will change hands for. However, another misprinted £2 coin, in which the words 'Am I Not A Man And Brother' are printed upside down, is changing hands for as much as £385.

Change Checker says it has 24 of the misaligned coins - but warns collectors to be careful about buying them on sites such as eBay.

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"Are you seeing a genuine Inverted Effigy or just a coin that someone has rotated in a photograph to make it look inverted?" it says.

You might laugh at this idea, but when a prankster recently put an 'upside down' fiver up for sale, it attracted a number of bids. Described in the listing as 'the only known upside down £5 note in existence', it was later withdrawn from sale.