One in five buses were found to have at least one serious defect affecting their roadworthiness in random spot checks across Northern Ireland.
The startling figures were revealed in the 2012/13 Driver and Vehicle Agency Heavy Goods Vehicle, Taxi and Bus Compliance Surveys.
Just under 450 buses were checked of which 21% had at least one serious defect, with 16% committing a serious traffic offence.
However, just under a third of buses were deemed unroadworthy in the random government spot checks, which were carried out on public and private-hire buses across Northern Ireland.
Figures released by the Department of the Environment (DOE) also reveal that nearly one-third of taxis did not meet basic compliance, with both buses and taxis now falling short of government targets.
There are now calls for urgent action from Environment Minister Mark H Durkan, who described current levels of compliance as unacceptable.
UUP South Antrim MLA Danny Kinahan, who sits on the Assembly's Education Committee, said he was "more than shocked" by the compliance rates.
"If we have children and people travelling on these sorts of transport, that is something that needs to be dealt with urgently," he said.
West Tyrone Sinn Fein MLA Michaela Boyle, who also sits on the education committee, added: "I know the DVLNI has made a number of changes to their checks so more things are tested, and this needs to be considered.
"But no car or vehicle should be on the road if it is not roadworthy, especially if it is bringing children to school or people to events or their work.
"Am I concerned? Yes I am concerned."
The annual report is based on quarterly, random checks of public and private buses and taxis. It reveals that:
* 30.4% or 136 buses checked were issued with a prosecution or prohibition notice regarding a serious roadworthiness defect and/or serious traffic offence.
* One in three of the 447 buses checked were found to be unroadworthy.
* 21% of buses checked had at least one serious defect, with 16% committing a serious traffic offence.
* Of those deemed not roadworthy, 21 (of 146 assessed) were school buses.
* 64 bus operators and drivers were issued with a prosecution notice in relation to serious defects, with another 42 operators facing legal proceedings in relation to traffic offences.
* Of the most common serious road defects detected, 15% were faults with brakes, while just under a quarter were related to lights or signals. Nearly a third of the faults were classified as other mechanical defects.
* Serious traffic offences detected included faults with fire extinguishers (14.6%) and the number of hours a driver has been working (22%). Other faults included defects with buses' brakes, exhausts, steering, chasis, visibility, tyres or suspension.
Problems with the vehicles' wheels or fire extinguishers were the basis of 12% of verbal warnings issued following a spot check, and vehicles' emergency exit defects led to 8% of subsequent verbal warnings issued.
The number of unroadworthy buses on our roads has increased since the last DVA report in 2012, which found non-compliance among 22.6% of buses, 7% less than 2012/13. Serious traffic offences are also on the up, with 5% more between 2012/13.
Taxis meanwhile showed increased levels of non-compliance, with nearly a third, or 169 of the 543 checked, deemed unroadworthy.
Nearly half (264 taxis) had at least one defect impacting on its roadworthiness.
Just over a quarter had at least one serious defect affecting the vehicle's roadworthiness and leading to prosecution or the issuing of a prohibition notice.
Around 70% of all buses and taxis checked were found to be roadworthy, the survey added.
Compliance has meanwhile increased among heavy goods vehicles (HGVs).
Translink moved quickly to assure the general public "that bus travel is very safe".
A spokeswoman said: "96% of all Translink buses pass their PSV Re-certification to assess their roadworthiness first time.
"This is extremely high and compares favourably with other large operators in GB where the first time pass rate is 93%.
"We are aware Translink vehicles were included in the DOE survey; however at this stage we have not had sight of the results.
"We can confirm that Translink received no prosecution notices.
"We work hard to maintain our good bus safety record.
"Travelling by passenger transport is seven times safer than travelling by car."
However, Environment Minister Mark H Durkan (below left), said current levels of compliance were unacceptable.
"This has potential implications for road safety and for the environment," Mr Durkan said.
"Standards still need to be improved across all strands of the industry and it is imperative that the reforms that are being taken forward for HGVs, taxis and buses are embraced by the industry."
He added: "My department will continue to work with the industry to improve the level of compliance by targeting the most non-compliant operators and drivers, rewarding those compliant operators and drivers and targeting the least compliant."
Shocking figures must be taken seriously and acted on
Most people waiting at a bus stop just assume that the vehicle that picks them up will be safe and roadworthy. The revelation that almost a third of buses checked had a serious defect that rendered them not roadworthy is shocking.
While some kinds of defect – such as a broken light – can occur at any moment, others such as defective brakes really ought to be picked up during routine maintenance.
In a worst-case scenario, a serious defect such as this could lead to a chain of events that culminate in a fatality.
For example, a bus whose indicator is not working could lead to a pedestrian stepping out to cross a side road, unaware that the bus is turning into it. Or a defective brake could result in a bus failing to stop in time when an elderly lady steps onto a zebra crossing.
When we also consider that, every day, thousands of our children are taken to school in buses we can see that these figures must be taken seriously.
While car owners are responsible for the roadworthiness of their vehicle, bus users rely on the drivers to ensure their safety.
One third is much too high to be the result of random chance, so it suggests that maintenance regimes need to be improved.
Perhaps funding pressures are leading to reduced maintenance frequency or a lack of funds for spare parts. If this is the case, managers need to carefully consider how the allocation of funds is having knock-on effects.
A thought too for the ever-increasing number of cyclists in Belfast. It will not be comforting for such vulnerable road users to learn that a third of buses, whose lanes they often share, have serious defects.
Given that the Department for Regional Development is currently considering letting all taxis use bus lanes too, the revelation that almost a third of taxis also had a serious defect or were committing a serious traffic offence ought to be of particular concern to those considering such a move.