A BBC Spotlight investigation has revealed that MOT emissions tests on diesel cars have not been carried out in Northern Ireland for 12 years, despite being legally required.
The Driver and Vehicle Agency (DVA) said its staff do a “visual test” and make a “personal judgement” about emissions.
Professor Robert Lee of Birmingham University, said failure to do the test means the government in Northern Ireland is breaking the law.
He added: “I think if you were a person that had a failure elsewhere in the UK on the basis of diesel smoke emission and you took your car over to Northern Ireland and had that vehicle tested in the testing station, and gained an MOT certificate, clearly you would take your car to wherever it was – Scotland or wherever it was – you came from and drive the car for another year.
“I don’t actually see what stops you as an individual doing that.”
Both EU and Northern Ireland domestic law requires the government to do an emissions test on diesels at MOT using a smoke meter.
The tests were introduced at MOT in Northern Ireland in 2006 but were suspended 12 weeks later due to concerns about health and safety in the testing halls – but they were never reintroduced.
The DVA’s chief executive, Paul Duffy said: “Resuming the metered smoke test for diesel cars will require a significant investment programme, which will also provide additional capacity to address the growing demand for vehicle tests across Northern Ireland and which is currently under way.”
A new centre in Belfast equipped to do the test is reported to open in 2021.