There's one in an old coat pocket or hidden in the back of the couch.
Approximately 169 million of the £ 1 round coins were not returned to the Royal Mint, nine months after they had ceased to be legal tender.  The £ 1 round coin has been replaced by the 12-sided version in October of last year, with millions of old coins that have been merged to create the new ones.
But while you can not spend the old currency, you can still take your bank and deposit it on your account
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The new £ 1 coin was introduced on 28 March 2017 to help repress counterfeiting, with one in 30 of the old version estimated to be false according to the Royal Mint.
Approximately 138 million round £ 1 coins have been merged and reused – to create about 1.5 billion new coins at the Royal Mint, based in Llantrisant near Cardiff, according to data from BBC Wales.
The new coin is described by the Mint as the most "safe in the world" and has a series of anti-counterfeiting details, including a hologram and micrometric letters inside both circles.
It also has material within which it can be detected digitally with the counting of coins or with payment machines
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A spokesman for the Mint said that he expected about 85% of the 1 billion £ 1 pound sterling – or 1.4 billion of them – to be repaid during the period of transition, based on the number of coins returned when the 50p coin changes in 1997-98.
"We do not expect all the £ 1 coins in circulation to be returned at the time of the mint transition," he said, adding the numbers returned were in line with expectations.
"Also, based on the returns of other demonetized currencies, we expect returns for many years to come when people find them."
Royal Mail refused to say how much money the euro coins melted 1 save in the production costs of the new £ 1 coin, saying that disclosure of information "could put us at a material disadvantage in any future negotiations with our overseas customers for the production of their coins".
According to the Royal Mint, the old £ 1 coin can continue to be deposited into a client's account, be it owned or personal, at most banks.
These include RBS, NatWest, Ulster, HSBC, Barclays, Lloyds, Santander, National, Clydesdale, Yorkshire Bank, Halifax, Bank of Scotland and The Post Office.