Guilder stamps were no longer accepted by the Dutch post office from November 2013 because of a massive forgery case, a PostNL spokesman confirmed on Saturday, following a report in the Volkskrant.

PostNL declared guilder stamps to be no longer valid 11 years after the introduction of the euro and said at the time this would make the processing of letters and parcels easier.

However, the Volkskrant says two men from Limburg had been selling guilder stamps – some of which were forged – to consumers and companies, allowing them to send post much more cheaply.

Justice ministry officials suspect the men of printing the stamps themselves, a charge which they both deny. They claim to have bought them from a third person, without knowing they were forged.

The two men, named as Jurgen L (41) and Dimitri W (40), have already been ordered to pay €2.6m compensation to PostNL in a civil case. However, the public prosecution department told the Volkskrant they hope to start a criminal case against the two men and six others before the summer.

It is not clear how much money is involved, but at least €2m worth of guilder stamps were used in 2012, the paper says.