375,000 cars are on the roads without a valid NCT, which is 170,000 above the norm for this time of year.
Operators 'Applus' will today apologise to customers for recent booking issues in front of an Oireachtas Transport Committee.
It will say that there's a number of reasons for the high figures.
Backlogs are partly down to a large number of motorists failing to show up for their tests and an ageing workforce.
The company will say around 2,500 people a week don't show up for their test.
Another 1,000 people cancel their appointment just before the test, leaving little time for the company to find a replacement.
In some parts of the country, the next available appointments aren't until September.
The National Car Test first began in Ireland in 2000, to test if vehicles are roadworthy.
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