The Metropolitan Police has uncovered £4 million worth of fake coins in a freight container, believed to be the biggest haul of fake coinage ever recorded in the UK.
The Met seized the £4.1m stock of counterfeit coins during raids in London, Hertfordshire and Essex.
Detectives said it was a "significant blow" to the crime gang behind the scam.
But officers said they believe some of the money produced may already be in circulation.
Three men, aged 52, 43 and 27, were arrested and are being held at a north London police station.
Detective Inspector Bruce South, from the Met's projects team, said: "This seizure is a significant blow to the network behind it; individuals clearly intent on undermining the UK monetary system by producing counterfeit currency on an industrial scale.
"It is yet another example of the work this team is carrying out on a daily basis to tackle organised criminal networks."
It is believed that there are roughly 1.5 billion pound coins in circulation across the UK, of which three per cent, or 45 million, are fake.