The proportion of untaxed vehicles used by evaders and falsely declared as off the road in Northern Ireland has risen significantly, the authorities said.
There were 27,018 reports of tax evasion in eight months last year but that number has been inflated by the same incident being reported multiple times, it is understood.
The Driver and Vehicle Licensing Agency (DVLA) said 12% of untaxed vehicles detected had been wrongly declared as off the road across the UK.
A spokesman said: "This is a significant rise from the previous survey in 2015 where it was just 1% and clearly shows that some people think it's OK to break the law and falsely declare their vehicle off the road yet continue to drive it while untaxed."
More than 98% of all vehicles on the road are correctly taxed, with UK-wide figures showing £6 billion a year collected in tax.
The DVLA said it clearly "shows that the overwhelming majority of motorists stay legal and tax their vehicles on time".
Vehicle Excise Duty evasion statistics published last year showed that 12% of untaxed vehicles spotted during the fieldwork were declared as off the road.
Motorists organisation the AA has suggested scrapping the paper tax disc may have contributed to evasion rates.
The DVLA spokesman said: "The fact is it's been three years since we got rid of the tax disc; this clearly isn't about people not knowing their legal responsibilities as more than 98% of vehicles are taxed correctly.
"This is about those people who are choosing not to tax and thinking they can get away with it.
"We are determined to crack down on all law breakers and will take tough action including clamping and impounding their vehicles."