Fuel laundering and smuggling by ex-IRA gangs and border criminals has been virtually wiped out, the taxman has claimed.
A Revenue official told the state's public spending watchdog that a survey this year of filling stations provided "authoritative evidence" that the apparent multimillion-euro trade is close to being put out of business.
"The fuel laundering problem has been effectively eliminated," the accounting officer told the Comptroller and Auditor General (C&AG).
The claim was made as part of the C&AG's audit of efforts by customs to tackle fuel laundering.
The watchdog said a series of initiatives led to a crackdown on the underground business and that no evidence of laundered fuel was found in tests on 197 licensed traders this year.
It also pointed to the success of a joint exercise by Revenue and the UK's Revenue and Customs to introduce a new fuel marker on both sides of the border in April last year to make it harder to launder diesel.
Despite the claim from Revenue, chairman of the Public Accounts Committee Sean Fleming said: "Fuel laundering is a major problem and there is a substantial loss to the state in taxes foregone.
"Shockingly, there is no revenue estimate of the loss to the Exchequer as a result of fuel laundering. There has to be a better way of dealing with this issue and we will consider all options in relation to these taxes."
The C&AG was told by Revenue that it does not estimate how much the black market operations were costing.
The watchdog urged it to consider changing its stance to measure the scale of the problem and the impact of its crackdowns while also understanding losses in the tax system.
The Revenue claim came months after an independent report estimated that the Exchequer in Dublin lost €239 million last year through illegal laundering.
The C&AG said new licensing of fuel traders and electronic reporting have made it more difficult for gangs to infiltrate markets and that reckless trading provisions introduced in 2013 are a further deterrent for suppliers from supplying such fuel.
Revenue also said a national sampling survey of fuel in September last year found just 2.9% of samples test positive for the new marker and a random sampling exercise in January this year found no evidence of the new marker.
Another major issue is the environmental impact.
Revenue said it has cost €6.7 million since 2008 to clean up more than 1,000 dumps of toxic sludge from illegal fuel laundering plants, nearly all of which were in Louth and Monaghan.
The watchdog also revealed 15 filling stations were shut down last year as part of the wider Revenue crackdown on the sale of illicit laundered fuel.
From 2011 to 2015 a total of 149 service stations were forced to close their gates because they were unlicensed or breached the terms of their licences.
The report showed 47 commercial seizures of laundered fuel were made last year involving 215,132 litres.
No laundering plants were uncovered but there have been 35 in the last five years.
