Three Belgian companies are being prosecuted for exporting chemicals to Syria, one of which could be used in the production of sarin gas.

The firms say they acted with the seeming consent of Belgian customs between 2014 and 2016, but were subsequently alleged to have failed to apply for the permits that would protect them from falling foul of a current EU ban on the exports.

According to a UK research group, Bellingcat, and a German non-profit organisation, Syrian Archive, Belgium is the only EU country to have exported a chemical called isopropanol since July 2013, when strict EU controls came into force.

The customs investigation into the firms appears to have been initiated by the Dutch authorities.

Isopropanol, more commonly known as rubbing alcohol, is found in disinfectants, cleaning agents, paints and varnishes, but it is also a major component of the type of gas used in recent suspected chemical attacks by the Assad regime.

Following the death of more than 80 people in an attack on the rebel-held town of Khan Sheikhoun in north-western Syria on 4 April 2017, the French government said that “since 2014 it has been established that Syria has been attempting to acquire dozens of tons of isopropanol”.

Between May 2014 and December 2016, there were 24 deliveries to Syria from the three companies, who were working together, containing 168 tonnes of isopropanol, 219 tonnes of acetone, 77 tonnes of methanol and 21 tonnes of dichloromethane, according to a court summons.

Sarin, which has been banned since the 1993 chemical weapons convention (CWC), is made through a reaction between isopropanol and methylphosphonyl difluoride.

The trial of the Flemish firms – AAE Chemie Trading, Anex Customs and Danmar Logistics – along with a managing director and a manager, is set for 15 May in Antwerp.

The irregular export of goods subject to permits can be punished with a prison sentence of between four months and five years.

A spokesman for the Belgian federal authorities said: “Customs have established infringements of the law concerning the import, export and transit of goods. The established facts were the subject of a criminal customs investigation and the prosecution was initiated at the end of March at the criminal court.”

The spokesman added that they had no reason to believe the chemicals in question had been used in the production of weapons.

AAE Chemie confirmed that it had been exporting isopropanol at a concentration of 95% or more to Syria, but said that it had been trading with private partners in the country for 20 years, and “none of those companies appears to be on any suspicious list”. The company said it had not been aware of the need for a licence for the exports and, before being summoned to court, believed it had come to a settlement with the authorities.

A representative of Anex Customs and Danmar Logistics told Knack magazine in a statement: “My customer [AAE Chemie] has been exporting chemicals to various paint factories and leather processing factories in Syria, Lebanon, Jordan for 15 to 20 years.

“Since the war in Syria the risk analysis of customs for export to that country has been set for scanning, physical checking and checking of the documents.

“It is therefore not the case that we have hidden ourselves and/or have taken illegal actions. Also in contacts between AAE Chemie and high customs officers, no one pointed out that something was wrong. Never was there even one export refused.”

• This article was amended on 19 April 2018. An earlier version referred to a global export ban, in fact it was an EU ban. That version also mentioned the coming into force of a prohibition from the Organisation for the Prohibition of Chemical Weapons, whereas these were strict EU controls.