The toy cash was sandwiched between real euro notes in piles of money as high-value deals took place.
On other occasions the word ‘Monopoly’ or ‘facsimile’ was covered by paper bands used to hold the bundles together.
As a result, dealers were duped despite using a pen or a machine to count the money, which resembles authentic currency but is lighter, a court heard.
They only discovered they had been tricked after transferring the valuables to the crooks who had fled.
The ‘ingenious’ gang even built their own desk where a man could hide and swap the genuine and fake notes in the drawers as the deals went on around him.
The biggest single con saw celebrity jewellery dealer John Calleija - whose red carpet clients include Zara Phillips and Colin Firth - defrauded of £6.1 million.
Mr Calleija, who owns a shop in the Royal Arcade on London’s Old Bond St. was told an investor had five million euros to splash on diamonds.
Gang leader Gianni Accamo, posing as a gem expert, went to inspect his stock of precious stones and a deal was struck at a Covent Garden hotel.
Mr Calleija and a bodyguard then used his cash counting machine to sift through bundles of 500 euro notes, totalling 7.7 million euros, with no hiccups.
But when the pair turned their backs to put the machine away, the tricksters switched the real euros for bundles of counterfeit cash.
The notes - which had “facsimile” written down the middle - were sandwiched between authentic bills.
The diamonds were then collected from Mr Calleija’s shop and it was only when the gang were long gone that he discovered he had been scammed.
David Hughes, prosecuting, said: “You can imagine the horror when he goes to check the cash.
“It is immediately apparent it is not the same that he had counted in the hotel. It’s counterfeit.
“At some point during that fatal turn around to put that cash counting machine away, the real money has been switched, the fake money has been substituted and the criminals had disappeared with the diamonds.”
Similar scams involving Monopoly euros were pulled off in Bristol, Wiltshire and Leeds.
Each bill had the word ‘Monopoly’ or ‘facsimile’ written on it in large letters but this was hidden by a band being used to hold the bundles together.
In Bristol they committed a £420,000 fraud involving Wales-based jeweller Jack Cohen who struck a deal to sell watches and diamonds.
Members of the group met with him at the Marriott Hotel, Bristol in August 2014 and paid him real euros for the watches and a banker’s draft for the diamonds.
After a problem was discovered with the cheque, it was agreed the jeweller would be given euros to cover its cost until it was corrected the next day.
He was handed bundles of Euro 100 bills totalling more than Euro 500,000 - and didn’t notice they were board game notes until later, Bristol Crown Court heard.
The notes were printed in such a way that the ‘Monopoly’ wording was hidden by some genuine outer notes and the paper bands around them.
It is thought they were printed like this so that if the offenders were caught with them they could say they were obviously fake and couldn’t be used to trick anyone.
In Leeds jeweller Marvin Santos lost nine expensive watches and a diamond linked gold chain worth £200,000 to the con gang in Autumn 2014.
Accamo, 44, Dusica Nikolic, 45, Dragoslav Djordjevic, 44, and his sons Juliano Nikolic, 26, and Bruno Nikolic, 27 were jailed for a total of 22 years on Friday.
Serbian-born Djordjevic, of Nottingham, also admitted a £250,000 theft of jewellery, watches, diamonds and sterling from a seller in a separate scam in Wiltshire.
He was jailed for seven years and four months after admitting conspiracy to commit fraud in Bristol, theft in Wiltshire and money laundering.
Accamo, of no fixed address, was convicted in May of the Leeds scam but a jury failed to return a verdict on his part in the Bristol job.
He was jailed for six years on Friday for his role in the Leeds and London frauds after previously pleading guilty to his part in the huge London scam.
Djordjevic’s son Juliano Nikolic, of Nottingham, admitted conspiracy to commit fraud in Bristol and money laundering and was jailed for four years and four months.
His brother Bruno Nikolic, of Gainsborough, Lincs., admitted conspiracy to commit fraud in the Bristol scam and was jailed for 22 months.
Dusica Nikolic, of Nottingham, had previously admitted concealing criminal property - money laundering and was jailed last September for two and a half years.
Judge Julian Lambert told them: “The fraud operated required you men to establish confidence in your victims so it was thought you could be trusted.
“You operated as a crime group, using reconnaissance and meticulous planning.
“It required considerable ingenuity and effort to establish dishonest ends.”
Most of the fraudsters were arrested in November 2014 at an address in Nottingham, where police found counterfeit notes and £150,000 in sterling, euros and dollars.
Officers also retrieved stolen jewellery hidden under soil in plant pots and a desk cabinet with a false front, where a person could have hidden to switch notes.
The convicts now face confiscation proceedings in court under the Proceeds of Crime Act 2002.