375,000 cars are on the roads without a valid NCT.
Operators Applus will today tell the Dáil Transport Committee the backlogs are partly down to a large number of motorists failing to show up for their tests and an ageing workforce.
In some parts of the country, the next available appointments aren't until September.
The company will say around 2,500 people a week do not show up for their test.
Another 1,000 people cancel their appointment just before their test.