Royal Mint admits faulty £1 coins are in circulation
Some new £1 coins - introduced into circulation last month as the "most secure coin in the world" - may be defective, the Royal Mint has admitted.
It said that a "small number of coins" were affected when they were struck at a rapid rate during production.
The Royal Mint is striking 1.5 billion new 12-sided £1 coins, introduced to help crack down on counterfeiting.
Out of shape versions of the coin, considered collectors' items, are appearing on internet auction sites.
The Sun newspaper has spoken to a number of people reporting warped coins, although doubts have been raised about the cause of reported highly-damaged coins.
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A spokeswoman for the Royal Mint, which produces three million new £1 coins a day at its headquarters in Llantrisant, South Wales, said: "The Royal Mint produces around five billion coins each year, and will be striking 1.5 billion new £1 coins in total.
"As you would expect, we have tight quality controls in place, however variances will always occur in a small number of coins, particularly in the striking process, due to the high volumes and speed of production."
The Mint has claimed the new £1 is the "most secure coin in the world", replacing the previous £1 coin, of which about one in 40 are thought to be fake.
'I got a badly minted pound coin from a Chinese takeaway'
Kristofer Bramwell from Kegworth in Derby tells the BBC:
"I paid for my food with a £20 note and it was in the change. I received it with a pristine minted new 1 pound coin.
"After getting home and eating the takeaway, I took the change out of my coat pocket and I found the badly minted pound coin.
"The edge of the £1 coin is partially 12 sided and rounded where it clearly has not been pressed correctly by the machine."
Philip Munsell, director of Coin News magazine, told the BBC that only a tiny minority of these new coins would be affected, but that fact made them highly collectable.
"The thing about the Royal Mint is that they are very, very good with their quality control, so therefore if you have something that has passed through without being quality checked then it is likely to be collectable, it is likely to therefore be worth more than one pound," he said.
He said that if the middle of the two-piece £1 coin was missing, it would not get through the quality checks.
The new coin has a string of anti-counterfeiting details, including material inside the coin itself which can be detected when electronically scanned by coin-counting or payment machines.
Other security measures include an image that works like a hologram, and micro-sized lettering inside both rims.
However, it is not the first currency launch to have hit problems in recent months.
Vegans and some religious groups have voiced concerns about the new polymer £5 note introduced in September last year, as it contains a small amount of tallow, which is derived from meat products.
Meanwhile, the old £1 coin remains as legal tender until 15 October this year, after which shops are under no obligation to accept it.
The new £1 coin: Vital statistics
Thickness: 2.8mm - thinner than old coin
Weight: 8.75g - lighter than old coin
Diameter: 23.43mm - larger than old coin
Number to enter circulation: 1.5 billion - about 23 per person. Old £1 coins will be melted down to make new ones
Outer ring: gold-coloured, made from nickel-brass
Inner ring: silver-coloured, made from nickel-plated alloy