More than 1,000 litres of illegal fuel have been removed from a filling station in Co Antrim.
Two diesel forecourt pumps and documents were also taken away by Revenue and Customs officers near Ballymoney.
Mike Connolly, assistant director for specialist investigations, said: "This filling station was operating at an unfair advantage over legitimate traders in the area by selling illegal fuel at a reduced rate.
"Legitimate businesses are already facing difficult times and it is wrong that they should be undercut by law breakers."
In an unrelated operation near Rathfriland, Co Down, on Tuesday, customs officers uncovered a diesel laundering plant capable of producing nearly two million litres of illegal fuel a year, evading almost £1.2 million of duty.
Storage tanks, 30kg (66lb) of a bleaching agent, 1,300 litres of laundered fuel, 1,000 litres of toxic waste - the bi-product of the laundering process - pumps and equipment were removed from the site.
Mr Connolly added: "Fuel laundering is unregulated and dangerous. 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.
"Laundered diesel may save you a few pence at the pumps, but the true cost is far from a bargain."