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Is that £1 coin in your pocket worth £25? Check the 'scarcity index' to see if you're minted

With just a few months to go before the round £1 coin is withdrawn from circulation, it’s time to check your pockets.

That shiny £1 coin you left in your shorts from last summer could be worth a tidy little sum.

Coin collectors are racing to complete the full set of 24 individual designs for the old coin – with the most rare expected to fetch as much as £25 each.

The Bank of England is introducing a 12-sided coin next month, and has already begun to phase out the old round design.

The Edinburgh City design on the £1 coin has been identified as the most rare

The £1 coin has been in circulation since 1983. During that time a total of 2.2 billion £1 have been struck for circulation. But they are not all still in use.

The last available figures for coins in circulation, published by The Royal Mint for 2014, suggest that 1,553,000,000 £1 coins are in circulation.

In other words, 650 million of the coins struck no longer circulate, presumably withdrawn over the years as worn or damaged.

Three of the most scarce £1 coins in circulation

According to money specialists ChangeChecker.org: “The majority of those 650 million coins are from the early issuing years, meaning that although some of those years may have high mintages, the actual number of coins available to collect from your change is far lower.

“In fact our research suggests that only a little more than half of the early years’ £1 coins are still in circulation. Far fewer if you’re trying to secure one in good collectable condition.”

The ChangeChecker £1 coin scarcity index

ChangeChecker has produced a scarcity index, ranking the various £1 coin designs on their rarity.

The highest scoring coin – the Edinburgh City £1 – it is already achieving prices of between £10 and £15 on eBay.

However, things can very quickly go mad. For example, since the Royal Mint confirmed the 2009 Kew Gardens 50p as the smallest coin mintage in circulation, online prices have regularly achieved £100 (200 times its face-value).

The round pound ceases to be legal tender on October 15 this year, so people have until then to either spend them or exchange them at a bank after that.