Customs officials have raided two fuel laundering plants capable of making 13 million litres of illegal fuel a year.
The plants, near Armagh and Cullaville along the border were raided simultaneously on Tuesday but the two operations are not linked.
Claire Merrills from HM Revenue and Customs said this would result £8m of lost revenue for the tax-payer annually.
"The plant on Moy Road is capable of laundering 2,500 litres of fuel a day."
She added: "The laundered diesel is taken to local filling stations and people who have no idea what's in it or what effect it has are using it to fill their cars," Ms Merrills added.