The number of fake £1 coins in circulation has doubled in the last five years and now stands at more than 30 million, the BBC has learned.
This means one in every 50 pound coins in circulation is counterfeit.
Experts believe the fakes are being produced by organised criminal gangs using specialist machinery.
The Royal Mint said it was illegal to make or use counterfeited coins and said people must hand them in if they thought they had one.
Although relatively easy to detect, their sheer number creates problems for shopkeepers, taxi drivers and other small traders who deal in cash for many of their transactions.
According to figures obtained by the BBC from the Royal Mint, random sampling tests carried out earlier this year showed that approximately 2% of pound coins are fake.
Confidence in coins
The Royal Mint described the counterfeiting rate as "a comparatively low incidence of counterfeit coins by international monetary standards."
However, other experts disagree.
Robert Matthews, was the Queen's Assay master until he retired to become a coin consultant four years ago. He said confidence in coins collapsed in other countries when forgery rates reached similar levels.
He said: "In 2004, people started refusing to take the South African 5 Rand coin, due to concerns about the number of counterfeits, and eventually the coin had to be redesigned and re-circulated.
"Independent surveys showed the number of counterfeits to be 2% - the same as we've got here - and I'm worried that if we're not careful the same thing will happen to the pound coin."
SPOTTING A COUNTERFEIT COIN
Look at the edge - on fake coins lettering will often be indistinct, or the wrong typeface
Hold the coin so the Queen's head is upright and facing you. The pattern on the reverse side should also be upright
The colour can be a give-away, although real coins can become discoloured in circulation
Most fake coins often won't operate automated vending machines
Since its introduction in 1983, there have always been attempts to forge pound coins, but the numbers were relatively small.
The last time forgery figures were released by the Royal Mint, in 2003, the number of fakes were estimated to be 1%. This suggests the number of counterfeit coins has doubled in the last five years.
The euro is a far more "secure" coin when compared to the pound. In 2006 it was estimated of 13 billion circulating coins (50c, €1 and €2), around 10 million were fakes, which works out at less than 0.1% of the total.
Mr Matthews said: "The EU has been openly directing that Euro coins have to be authenticated after withdrawal and before reissue without creating public panic. I know of no public moves of this nature in the UK."
He said he was worried that in Britain, dealing with the fakes was falling between Royal Mint, HM Treasury, the banks and the police, with no one taking direct responsibility.
"And, at current detection rates, it'll take 120 years for the counterfeit coins that are circulating right now to be withdrawn," he added.
The Royal Mint said in a statement: "We track the counterfeit rate through regular surveys in the spring and autumn every year. The survey consists of taking a random sample of coins from across the country, and subjecting them to individual analytical inspection.
"It is a criminal offence to make or use counterfeited coins. Any member of the public who suspects they have a counterfeited coin should not attempt to spend it."
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