Three people have been arrested after Customs officials swooped on illegal filling stations.
Two men and a woman were detained and more than 2,000 litres of illicit fuel, forecourt pumps and other equipment seized during the large scale operation in Belfast and Co Armagh to tackle suspected fuel fraud.
John Whiting, assistant director criminal investigation, HM Revenue and Customs, said: "The huckster sites we dismantled today were operating at an unfair advantage over legitimate filling stations by selling illegal fuel at a reduced rate.
"It is wrong that honest businesses should be undercut by criminals and we are targeting those individuals and groups doing most economic harm in our communities."
Two filling stations in west Belfast and Lurgan were removed. A computer, documents and business records were also recovered after Customs officers and police searched three houses.
The three people arrested were being questioned by HMRC and a spokeswoman said investigations were continuing.
Mr Whiting added: "We continue to work closely with our colleagues in the Organised Crime Task Force (OCTF) to stop the damage this criminal activity causes."
In unconnected activity, a Co Tyrone man was arrested after the discovery of a diesel laundering plant in Coalisland. He has since been released pending further enquiries.
The plant, hidden in a shed at a private address, was capable of producing 1.4 million litres of illicit fuel a year, evading almost £950,000 in lost revenue.
More than two tonnes of toxic waste, 3,000 litres of illicit fuel, pumps and equipment were removed from the site.