Kent Police has uncovered an alleged money laundering ring worth a “staggering” £5.2m in £20 notes.
When the in investigation was launched the Bank of England knew of at least £152,000 of counterfeit cash in circulation.
Chief constable Alan Pughsley told a meeting on Wednesday: “An operation commenced in January with regard to intelligence from the organised crime group counterfeiting currencies.
“Lots of work then commenced over a period including covert and overt work building an intelligence picture until May, where we believed we had identified the premises and believed there was a print run live at that time.
“We went to the address and found two individuals there at the time literally hands on in the middle of counterfeiting a number of £20 notes.”
At least five people have been arrested as part of the alleged conspiracy to print fake notes at a business premises in Beckenham.
Mr Pughsley added: “This is, according to the Bank of England, the biggest ever seizure of £20 notes.
“It would have had significant damage to the UK economy."