The public prosecution department has written to 22 companies which sell a key ingredient in the party drug GHB to say they face legal action unless they stop.

The traders sell gamma-butyrolactone, the raw material for GBL which was placed on the official list of hard drugs last year.

Traders can be jailed for up to six years if they cannot prove the trade is legal. The chemical is also used in stain-removers and solvents.

The public prosecutor is currently preparing a case against a trader arrested in 2012 with 42,000 litres of the chemical. He was arrested again earlier this year with 5,200 litres.

Last week a trader was found guilty of selling the chemical for the illegal drugs trade, the department statement said.