Two fuel-laundering plants capable of producing 18 million litres of illegal diesel a year have been raided in Co Armagh.
The huge volume of fuel the unconnected operations were capable of making was enough to evade £12 million in duty per annum, according to HM Revenue and Customs.
Officials from HMRC accompanied police officers as they entered commercial premises in Crossmaglen on Tuesday, but details were only released on Friday.
The laundering plant they uncovered was capable of producing over seven million litres of illicit fuel a year and evading almost £4.7 million of duty.
During the searches, officers found a newly made concrete pen containing thousands of litres of toxic waste, the by-product of the laundering process. Four HGVs, all of which had been adapted to transport the laundered fuel, tanks, pumps and equipment were removed from the site.
HMRC also announced that last Friday they discovered an unrelated laundering plant capable of producing nearly 11 million litres of illicit fuel a year at a cost of around £7 million in lost revenue. It was uncovered when customs officers searched agricultural outbuildings at a private residence in the Derrynoose area of Co Armagh.
Two HGV vehicles, 21,000 litres of illicit fuel and associated equipment were removed from the site.
HMRC also seized a 40ft curtain-sided trailer, containing two 15,000-litre tanks, recently adapted as a mobile laundering plant.
Mike Connolly, assistant director of specialist investigations at HMRC, said: "HMRC takes the dangerous and illegal practice of laundering fuel very seriously.
"The by-products from the laundering process can cause severe damage to the environment with taxpayers and local ratepayers having to pay for the clean-up and disposal costs."