There are 375,000 cars without a valid NCT on the road.
In front of the Oireachtas Transport Committee today, operators Applus will apologise to customers for recent booking issues.
In order to determine whether vehicles are roadworthy, Ireland introduced the National Car Test in 2000.
Currently, 375,000 vehicles on Irish roads lack a valid NCT, which is 170,000 above the norm for this time of year.
Representatives from the operator of the NCT Applus will today tell an Oireachtas Transport Committee that there are a number of reasons for the high figures.
A large number of drivers failing to show up for their tests and an ageing workforce are two factors contributing to backlogs.
The company will say around 2,500 people a week don’t show up for their test.
Just before the test, another 1,000 people reschedule, giving the business little time to find a replacement.
The earliest appointments aren’t available until September in some areas of the nation.