A rare 20p coin that may be the only one of its kind could sell for as much as £1,800.
The unique coin was struck on a planchet – a plain metal disc – from a foreign country in 1990 instead of being produced using a bronze metal blank like most 20p coins made that year.
Collector Alun Barker is believed to have purchased the coin on eBay for £50 in December 2017.
It is now being auctioned by Tennants Auctioneers in Leyburn, North Yorkshire, on 9 August.
It was originally listed as a 20p struck on a 1p coin planchet, but experts have confirmed its true metallic makeup.
In January 2018, the Royal Mint verified the coin as authentic.
At the time of buying the coin, Barker, of Hull, said he was “over the moon” when he discovered how unique it was.
He said: "I've been told that if I want to sell it I should put a reserve on it at auction for £2,200.
"When I took it to be valued they said 'the sky is the limit' because it's the only one.
"Everyone I've spoken to has never seen anything like it. I'm still taking it in. The magnitude of it is amazing."
A Tennants Auctioneers spokesperson added: "This 1990 20p has been struck on a copper-plated steel blank intended for one of the other countries the Royal Mint produces coins for.
"It's a spectacular error and the only one of its kind known."
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A 1966 Pattern Crown featuring a photo of Prince Phillip and Queen Elizabeth will also be going under the hammer on 9 August.
It has a striking image of Britannia hurling thunderbolts on the reverse and is estimated to fetch £1,000 to £1,500.
Meanwhile, a seller in Glasgow auctioned off a 2019 Paddington Bear design 50p coin on eBay for an impressive £1,500 in February.
Another sought-after coin among collectors is the Kew Garden 50p coin.
This coin was created to commemorate the 250th anniversary of the famous botanical gardens in 2009, and only 210,000 were ever made.
Recently, a seller on eBay managed to fetch over £700 for one of these rare coins.