More than a billion brand new £1 coins are due to be released on 28 March. The 12-sided coin, described by the Royal Mint as “the most secure coin in the world”, will replace the traditional circular £1.

With just under five weeks to go, the Treasury and money experts have urged the British public to spend or exchange their round pounds before they cease being legal tender. However, before rushing to the bank, money specialist website Change Checker is advising customers to check their £1 coins carefully.

They could be sitting on a small fortune.

That’s because as the round £1 coin disappears from our change forever, collectors are desperate to complete a collection of all 24 designs.

In its “Scarcity Index”, Change Checker has identified 24 of the rarest £1 coins to have ever circulated the UK.

Some are worth as much as £50 each.

Coins scaled at 100 are the most scarce (Change Checker)

Scaled from 100 to 1, the scores represent the relative scarcity of each coin, with 100 being the most scarce.

The rarest coin on the index is the Edinburgh, which is already achieving prices of between £10 and £15 on eBay and could soon be fetching between £25 and £50, according to the website.

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The 2011 Cardiff City £1 coin – second on the scarcity index – is already selling for £20 on eBay with Change Checker predicting its value will climb.

The £1 coin was introduced in 1983, with a total of 2.2 billion having been struck for circulation since, according to Change Checker.

The last available figures, published by the Royal Mint in 2014, suggested that 1.55 billion £1 coins are in circulation.

Last year, a 2p coin sold for almost £1,400 at auction because it was silver instead of the more conventional copper.

Meanwhile, the Bank of England’s first ever polymer notes were selling for up to £800 on eBay or more than 160 times its face value.

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The website also gives collectors tips to find all 24 circulating £1 coin designs before they are replaced.