Ninety thousand litres of fuel has been seized in a cross-border operation across counties Armagh, Antrim, Down, Louth, Donegal and Cavan in Northern Ireland.
Customs officers from both sides of the border were involved and one man was arrested in Northern Ireland. The raids were part of a crackdown of the sale of illegal fuel from retail outlets.
Tanks, pumps and fuel pods together with six adapted vehicles were seized and two UK registered vehicles were impounded.
Mike Connolly, of Customs and Revenue, said: "By working together in partnership with our colleagues in the Republic of Ireland, part of the Cross Border Fuel Fraud Enforcement Group, we can stop substantial amounts of harmful diesel entering the legitimate fuel market.
"Products of this nature are being sold at a reduced price and motorists may think they are getting a bargain because it is cheaper, but I would caution them to think again as this type of fuel will cause damage to your vehicle."
Almost 62,000 litres of the fuel which was recovered was found in Northern Ireland and 29,000 litres in the Republic.