The number of fuel laundering sites in Northern Ireland shut down by HMRC has more than doubled in the last five years, new figures show.
Some 38 operations were dismantled in 2013/14 compared to 16 in 2009/10, 20 in 2010/11, 29 in 2011/12 and 22 in 2012/13.
Separate numbers show the number of convictions for fuel fraud in Northern Ireland increased from four in 2010/11 to nine in 2013/14.
Both sets of figures were published in reply to written parliamentary questions submitted by SDLP MP Margaret Ritchie (South Down).
Treasury minister Priti Patel said HMRC was fighting fraud on a wide range of fronts, from special units performing thousands of roadside checks to raiding laundering plants.
She also highlighted the recent joint announcement by the UK and Irish Governments of a fuel marker, making it harder to launder and sell on without detection.
But the effectiveness of the marker was questioned by Northern Irish MPs during a Commons debate last week.
DUP MP David Simpson (Upper Bann) claimed the marker could be removed from the fuel, while h is colleague Ian Paisley (North Antrim) suggested the test had no roadside capability.
Shadow Home Affairs minister Jack Dromey also hit out at the Government, accusing it of opting for the "flawed" marker over a British product that was proved to be unmoveable but more expensive.
Andrew Murrison, for the Government, said he could not give an "absolute assurance" that any substance could never be removed.
But he insisted the marker was capable of being discovered and that it was an improvement on the current situation.