Customs officers seized 6,000 of illegal diesel at what they say was a "sophisticated" laundering operation in east Donegal.
A search was carried out on a service station by customs officers early yesterday, morning.
"Officers discovered a man in the process of discharging laundered diesel from a lorry into an underground 20,000 litre storage tank.
"They also found a sophisticated electronic switching system buried under a layer of gravel which which was designed to allow the diesel pumps to be fed from the concealed tank or from the regular tanks."
The steel-bodied lorry had been specially adapted with a "false floor" to allow it carry 4,500 litres of fuel. It was loaded with bags of peat moss as cover.
"Both the filling pipes and discharge pipes were well concealed. The lorry and approx 6,000 litres of fuel were detained and removed from the premises," the statement added.
A man has been questioned and a file will be forwarded to the Director of Public Prosecutions.
A Revenue spokesman said: "Laundering or 'washing' of red or green diesel defrauds the state of excise duty and VAT. People also need to be aware of the environmental and safety issues surrounding the laundering of diesel".
The spokesman urged members of the public to contact their nearest Revenue office if they know of any fuel misuse or suspicious activity.