Northern Ireland Customs officers seized more than 60,000 litres of illegal fuel from filling stations along the Border, they said tonight.
Working in a joint operation with the PSNI they made seizures at Newry and Warrenpoint, Co Down and Camlough, Co Armagh.
At the Newry garage they also seized a lorry which had been professionally adapted with a concealed tank for illegally transporting fuel.
At Camlough they also took away almost 25,000 cigarettes and a quantity of hand rolling tobacco.
A spokesman for Customs and Excise said all three filling stations had also failed to comply with regulations for VAT registration and registration for selling controlled oils.
The fuel seized was cheap agricultural red diesel which had been chemically "cleaned" for sale to motorists but which can damage engines.
Customs head of detection in Northern Ireland Mr Colin McAllister said: "These operations have stopped a substantial amount of harmful fuel entering the legitimate fuel market and damaging honest trade."
He said motorists needed to be aware of the full costs of buying cheap diesel.
"It puts honest filling stations out of business and robs the taxpayer of money for public services.
"The public also need to be aware of the environmental and safety issues surrounding the illegal transport of volatile fuels," he added.
A Police Service spokeswoman said no arrests had been made.