Almost 300 private bus operators in Northern Ireland could face prosecution after their vehicles failed to pass spot inspections.
The Driver and Vehicle Agency carried out 903 checks on buses and coaches over the past two years.
A total of 281 operators were reported with a view to prosecution.
The agency said the most common reasons for failure were because the vehicle was unroadworthy, inadequate insurance, or the driver had no bus licence.
Stephen Spratt, enforcement manager with the Driver and Vehicle Agency, said passenger safety was being put at risk by some operators cutting corners to save money.
"Whilst these statistics paint a dark picture, what I have to point out is that many of the vehicles stopped were targeted as a result of intelligence and complaints the agency had received," he said.
"We have to appreciate that these are hard economic climates out there where bus operators are competing for more limited business opportunities.
"That means there is an impact on the legitimate operators who may have less money to spend on the upkeep and the maintenance of their vehicles.
"Our highest successes have come from targeting venues where we have buses of revellers going to festivals or nightlife locations.
"We know cowboys operators go out there seeking to incentivise young party revellers who will be influenced by cheap transport."