The number of fake £1 coins in circulation appears to have doubled in the last five years, it was reported today.
The results of a sampling test by the Royal Mint, obtained by the BBC, show that up to 2 per cent of all the pound coins in circulation may be fake - approximately 30 million coins.
The last time sampling test results were released was in 2003 when the number of forged pounds coins was estimated to be 1 per cent.
The BBC said it means that one in every 50 pound coins in circulation is counterfeit.
Robert Matthews, formerly the Queen's Assay Master until he retired to become a coin consultant four years ago, said confidence in coins collapsed in other countries when forgery rates reached similar levels.
He told the broadcaster: "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 per cent - 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."
In a statement, the Royal Mint said: "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."
The BBC says that experts test for forgeries by studying lettering and the cross on the edge of the coin, whether the Queen's head is orientated the same way as the image on the reverse, and whether the marking is centred on the face.