The new, “most secure” 12-sided £1 coin entered circulation in March 2017, and until 15 October 2017, both the new and the old round £1 coin were used.

The move to launch a new £1 came as around one in 30 old round pounds was counterfeit.

According to figures obtained by BBC Wales, about 138 million round £1 coins have been melted down to create some of the 1.5 billion new ones at the Royal Mint.

It was expected that about 85% of the 1.7 billion round £1 coins – 1.4 billion of them would be returned during the transition period, based on numbers achieved when the 50p changed in 1997/98.

But around 169 million round £1 coins have not been returned to the Royal Mint, nearly 10 months after they stopped being legal tender.

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A spokesman for the Royal Mint said: “We do not expect that all round £1 coins in circulation at the time of the transition will be returned to the Mint.”

“Also, based on the returns of other demonetised coins we expect there to be some returns for a number of years to come as people find these.”

If you find an old £1 coin, you can’t spend it but you can take it to your bank to deposit into your account at the following: Bank of Scotland, Barclays, Clydesdale, Halifax, HSBC, Lloyds, Nationwide, NatWest, The Post Office, RBS, Santander, Ulster and Yorkshire Bank.

The Royal Mint said these banks are under no obligation to exchange coins with non-customers, and they could also impose deposit limits so it’s best to check before you go.