London: Some of the new one pound coins, launched 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 one pound 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.
A spokeswoman for the Royal Mint, which produces three million new one pound 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 one pound 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 one pound coin is the "most secure coin in the world", replacing the previous one, of which about one in 40 are thought to be fake.
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.
He said that if the middle of the two-piece one pound 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.
It is not the first currency launch in the UK to have hit problems in recent months.
Hindus, Sikhs, Jains and vegans in the UK have voiced concerns about the new polymer 5-pound note introduced in September last year, as it contains a small amount of tallow, which is derived from meat products.