Barcelona have temporarily suspended nearly 3,000 members who are suspected of fraudulently falsifying season tickets for the Clasico earlier this month.
A team of 120 people was deployed before the game against Real Madrid to detect how many counterfeit tickets had made their way on to the black market.
"It's the largest and most forceful action against fraud and the [illegal re-sale of tickets] that [Barca] have ever carried out," club spokesman Josep Vives said.
Barca have since identified and contacted 2,823 season-ticket holders who they believe handed over their details to a third party in order to obtain an illicit economic return.
The third party, the club suspects, would then have used those details to falsify tickets for the game, which, Barca estimate, has cost the club around €1.5 million -- around €500 per ticket.
Members who don't want to attend a home game have the option to return their ticket to the club, who would then be able to re-sell it -- usually for much more -- via their "free seat" scheme.
The majority of the tickets for the Clasico are believed to have been purchased by tourists for extortionate fees.
But despite spotting the falsified tickets on the day of the game, a 2-2 draw at Camp Nou, Barca did not prevent anyone from entering the stadium.
Vives explained that they would have been bought in good faith and that the club did not want any more problems around a game that had already been categorised as high risk.
Instead, a disciplinary committee will now seek to get to the bottom of the network that is re-selling the tickets.
In the meantime, the 2,823 members involved have been provisionally suspended, with their tickets for the final home game of the season against Real Sociedad on Sunday being passed on to some of the 10,000 people on the waiting list for season tickets.