Customs officials who smashed a multimillion-euro fuel laundering plant in the Irish Republic have claimed many more are operating in Northern Ireland.
A man has been arrested after the latest illegal fuel cleaning factory along the border was uncovered at Drumboat, Inniskeen, Co Monaghan.
It is believed the operation had the potential to cost the Irish taxpayer more than 10 million euro (£7.8 million) every year.
Revenue officers, backed up by an elite armed Garda squad, moved onto the site late last night after a lengthy surveillance operation, sparked by a curious official who noticed suspicious activity in the area months ago.
The plant, in a commercial yard close to the border with south Armagh in Northern Ireland, has the capacity to launder dyes out of about 20 million litres of oil every year. The dyes are used to differentiate fuels sold at lower taxes for certain industries, like agriculture.
It is estimated the Monaghan operation could have cost the public purse as much as 10.5 million euro (£8.2 million) every year in lost taxes.
A mobile oil laundry was concealed in an oil tanker at the plant.
Around 20 officials involved in the swoop seized 50,000 litres of laundered fuel, three oil tankers, two stationary tanks and other equipment associated with fuel laundering.
Toxic waste was also uncovered at the site.
It is thought the plant was in operation for about a year.
A 42-year-old man arrested during the operation is being detained at Carrickmacross Garda station.
Revenue's Sean Kelleher, the republic's chief customs enforcer along the border, said the operation has put a significant dent in the activities of fuel criminals but warned more plants were still operating in the north.
"Are there other plants similar to this around? The possibility is that there is, there are certainly quite a few plants operating north of the border as well," he said.
"This is first significant (plant) we have found in this State this year."
Mr Kelleher said a legislative crackdown in fuel laundering south of the border in recent years had helped stem criminality, but said leading figures in the black market trade were still able to operate.
"The main players in the illegality of fuel fraud, they are still able to source marked mineral oil for oil laundering purposes," he said.
"There are a limited number of plants operating and a lot of them, unfortunately, are operating north of the border.
"But the agencies north of the border are dealing with these plants and closing them down also."
Revenue officials believe fuel laundering gangs have control of a network of around 150 filling stations around the Irish Republic where they sell on the washed fuel.
The environmental cost of fuel laundering includes the clean-up bill for a sludge by-product left over by the process. It is estimated to runs into hundreds of thousands for local authorities forced to clean up the mess left behind.
Retail Ireland estimates that the illegal fuel trade costs the Irish Exchequer at least 150 million euro (£117 million) in lost taxes every year.
The lobby group said legitimate businesses are also forced to cut jobs and even close because they are undercut by the illegal fuel.