French police have smashed a vast cash-counterfeiting ring that printed over nine million euros and is described as the biggest ever in France, a police source said today.

Police raided the money-printing factory in Seine-et-Marne east of Paris on Tuesday and Wednesday, which had been produced more than 350,000 counterfeit 20 and 50 euro notes, the source said.

France's Central Bureau for Fighting Counterfeiting (OCRFM) has been working on the case since the counterfeit notes first appeared in circulation in 2007.

While officers had made dozens of arrests over several months, notably members of the traveller community distributing the notes, they had not until now found the master counterfeiter or the print works, the source said.

The print works was hidden on an industrial site in a small village between the towns of Meaux and Chelles. It produced the high-quality notes digitally rather than through the usual offset printing method.

Police eventually found the print works, tucked away behind a false wall and under a concrete slab after bringing in a surveyor.

Around 90 percent of the notes were put into circulation in France with the rest in neighbouring countries.

The master counterfeiter was described as a man in his 50s with a criminal record for counterfeiting, the police source said.

Between 30 and 40 counterfeiting operations are dismantled in France every year, making the country one of the most affected in the European Union.

But this is the first time that an operation of such size has been uncovered, police said.