The Road Safety Authority has admitted that 484 driving tests were cancelled last week for weather related reasons including unsafe conditions, ice and snow at test centres.
However, it has categorically rejected claims that driving tests were being "arbitrarily" cancelled due to low temperatures - but said any customers whose tests were affected by adverse weather conditions will be offered a re-test free of charge.
Earlier, the Unite trade union, which represents around 85 mainly self-employed driving instructors, accused the RSA of cancelling driving tests because the weather is too cold - at a time when essential workers are unable to secure tests due to backlogs.
Unite said the most recent figures suggested there was a backlog of around 64,000 driving tests, with waiting times soaring from six weeks a year ago to around 25 weeks at present.
The RSA acknowledged some tests were cancelled where the customer said they could not attend due to dangerous conditions,while some did not proceed as the tester was unable to conduct it because of "in vehicle conditions".
It noted that driver testers were undertaking driving tests "in extraordinarily difficult conditions in the current Covid-19 environment".
It pointed out that the test is conducted in an enclosed space for a period exceeding 15 minutes, where physical distancing is impossible.
To mitigate risk, it has put in place several controls to protect both testers and customers, including the introduction of "natural ventilation" to the car.
"Windows need to remain open to allow for circulation of air whilst driving. This is for the safety of both the tester and the customers, a responsibility the RSA takes very seriously," the agency says.
"Driving tests are only being cancelled where leaving the windows open and continuing with the test would mean that conditions would deteriorate to such an unacceptable level that it would be impossible to conduct a driving test," it said.
"Both the driver tester and the customer doing the test must be able to carry out the test safely. In conditions where a customer is trying to drive in excessively cold conditions and their ability to manage the vehicle is compromised is simply not a safe environment in which to conduct a test. It is a matter of health and safety which the RSA and its testers take very seriously," the RSA said.
Chair of Unite's Approved Driving Instructor Branch Dominic Brophy said: "We have been receiving multiple reports from ADIs that tests, for essential workers, are being cancelled due to the RSA testers feeling that it is "too cold" to slightly roll down the car windows (which is part of their Covid safety protocols) for a test."
"The backlog is big enough as it is, without cancelling tests for no good reason - especially for essential workers who we all depend on during this pandemic," he added.
Mr Brophy said the union had been seeking meaningful engagement with the relevant ministers and the RSA for months, adding that driving instructors and learners awaiting their tests had had enough of being messed around and treated as if they did not matter.
"If the RSA think it's appropriate to waste the time of essential and frontline workers during a pandemic, then something drastically needs to change," he said, and called on the Department of Transport to intervene.
Unite says it represents around 85 driving instructors, who are not public servants, but rather are either self-employed, franchisees or may work for a driving school.
A union spokesperson said many learner drivers hire their instructor's car for their test, mainly so that the instructor can be their accompanying driver, which is a legal requirement.
The Forsa trade union, which represents driver testers, acknowledged that a "relatively small" number of tests had been postponed in recent times "...because very bad weather including low temperatures and heavy rain and snow, made it impossible to conduct tests under these conditions."
The union said members were following RSA protocols based on professional advice that car windows must remain open throughout tests for public health reasons during the pandemic.
It noted that this was to ensure the safety of candidates and driver testers, who were in close proxmity - well within the two-metre social distancing regulation - for almost an hour during driving tests.
It said extraordinary measures had correctly been put in place as they had been across the economy and society to protect the health and safety of candidates and staff, and to contain the spread of the virus.
Forsa pointed out that in the UK and Northern Ireland, as well as a number of other European countries, driving tests had been completely suspended.
