BRUSSELS (AP) — Belgium's customs officers arrested 40 people Wednesday and seized tens of millions of counterfeit cigarettes in raids targeting illegal production sites across the country.

Belgium's finance ministry described the operation as the biggest in the history of the customs service.

The ministry could not provide an estimated value of the merchandise seized since a final inventory has yet to be done.

In addition to the cigarettes falsely branded as Marlboro, Richmond, Prince and Regina, customs officers also discovered tons of tobacco, cigarette filters, empty packs and glue. The operation targeted 10 different production sites.

According to Belgian authorities, illegal production of manufactured tobacco has been growing in the European Union in recent years.

Last month, the European Anti-Fraud Office (OLAF) supported Spanish and Portuguese authorities in dismantling an international criminal network specialized in the illicit production and sale of cigarettes. A total of 125 searches were carried out in Spain and Portugal during Operation Alecrín, with 29 arrests and over 2 million euros in cash seized.

Cigarettes seized in Belgium are mainly destined for the British and French markets, where excise duties are higher.