Northern Ireland’s Driver and Vehicle Agency has apologised after “technical difficulties” stalled online MOT bookings.
Many drivers trying to book their tests with the new online system in recent days were met with error messages.
It is now expected the system will be back up and running by Wednesday at 10am.
One frustrated motorist in Belfast was among those taking to Twitter to vent their frustration.
"You couldn’t make it up. The MOT booking system for Northern Ireland has been down since Friday, with a promise of being live at 10am this morning. Now this,” he said.
Speaking to the Evening Extra programme on the BBC, mechanic Bradley Spence from Good Autos in Saintfield said the delays were affecting his business.
"Already there’s a delay getting MOTs, we have customers who are maybe booking a month or two months in advance trying to get dates,” he said.
"Sometimes by the time they get a letter telling them their car is due for its test and they go on and book there’s no availability.
"This here’s just adding more fuel to the fire really.”
Mr Spence said there was now “a massive backlog” for MOT bookings, with customers potentially waiting up to two months unless there was a cancellation.
"Even cancellations are few and far between.”
On a typical day he said he had up to five cars in for MOT checks and the delays were also affecting their car sales.
"We’ve maybe sold a car and we can’t get the MOT, we can’t let that car go until it has a valid MOT.
"One last week there we were waiting maybe two weeks before that car could leave us.
"So that was a delay of two weeks waiting for that revenue to come in which isn’t good for business.”
A Department for Infrastructure spokesperson said: “The DVA is upgrading and improving its MOT and driving test booking system. The go live date for the new system was delayed in order to resolve some technical difficulties associated with the transition.
“Tests booked prior to the booking system being unavailable have been unaffected during this time.
“The new booking system for MOTs and driving tests will reopen tomorrow, Wednesday September 22 at 10am.
“The DVA apologises to customers for any inconvenience caused.”