The Government's new online MoT system has been branded a "debacle" after glitches left motorists in danger of being forced off the road.
Garage owners have complained they are having to turn away drivers because the system is running slowly or has crashed meaning they may be unable to issue valid MoT certificates.
The Driver and Vehicle Standards Agency (DVSA) said the system has been stuck by an "IT issue" and is asking garages to issue old-style paper MoTs and send the details on.
It comes as the DVSA has switched the issuing of MoTs to a cloud-based system, which uses apps to record the data on phones and tablets.
Paul Watters of the AA said: "We should have learnt from when the electronic MoT was introduced – you would have thought once bitten twice shy. It is a debacle.
"We want drivers to comply with motoring laws. The Driver and Vehicle Licensing Agency (DVLA) and DVSA are moving in the direction of cloud-based applications and I hope this is not a taste of things to come."
This computer glitch has affected an unknown amount of the 16,000 garages which have moved over to the new system.
Frustrated garage owners have complained they cannot get through to the DVSA's help desk or the software company Kainos which was contracted by the Government to carry out the upgrade.
Garage owner Howard Watts, who runs Riddelsdells Garage in Boxford, Suffolk, told the Daily Mail: "We have been having problems since last Thursday and just can't get the system to work.
"It means that we are turning away regular customers who need an MoT - and they shouldn't be on the road without one.
"There have been problems with every new computer system they bring in."
Drivers could risk being stopped by the police for not having a valid MoT even if it has passed so the contingency plans "must be honoured" so that a car can be taxed and "there is no embarrassment", according to Mr Watters.
He said: "We urge drivers to continue applying their cars for the MoT and to take the paper system.
"There will be a backlog and they should make sure they do everything to make sure that drivers and garages are not inconvenienced.
"We need to explore in depth why this project has gone wrong because we know that motoring services are increasingly being delivered in this manner. We support this as long as it works."
DVSA chief executive Alastair Peoples said: "We are aware there has been a delay in some MoT services due to an IT issue. We are working to urgently resolve this and minimise the number of customers affected.
"Contingency plans are in place. MoT certificates can still be processed manually and the online system is still functional.
"Almost 800,000 MoTs have already been recorded using the new online system and DVSA has received positive feedback from industry representatives. The system offers more flexibility for garages as it doesn't require specialist IT equipment, and has significantly lower running costs."
The cheapest cars to insure
Online MoT system branded 'a debacle'
Price new: £5,995
Dacia has made a big impact in the UK with its line of affordable motors.
The Sandero is the foundation of the brand's offering and is famed for being the UK's cheapest car.
With a group two insurance classification it's also one of the cheapest cars to insure right now.
The entry level Access model is sparsely equipped, but it has a practical interior, big boot and low running costs.
Price new: £8,060
The Seat Mii is a small city car which Auto Express praises for its spacious interior, good build quality and supple ride.
All models are in insurance group one, bar the group two Sport version.
The Mii is available in three-door or five-door styles, but both measure just 3.5 metres, making it perfect for getting around town.
Price new: £8,090
The Skoda Citigo was crowned Auto Express Best City Car 2013.
The magazine praised its low running costs and practicality as well as its grown up driving experience.
Like its Mii stable mate the Citigo has group one insurance in all apart from its top specification model, making it one of the cheapest cars to insure.
Price new: £8,265
The third of the Volkswagen's group of three city cars to make the list is the Up!
Auto Express says the Up! was 'born to rule' city streets with its small dimensions and lightweight body.
There's three versions of this fun city car; Take Up! has a group one rating as does Move Up! while the higher spec High Up! gets a group two classification.
Price new: £8,345
The Hyundai i10 is another car which can get you a cheap insurance quote thanks to a group one insurance rating across all the 1.0-litre petrol models.
Auto Express says the new 2014 i10 offers big car features in a small package. And for £8,345 you also get Hyundai's five-year warranty and roadside assistance package.
Price new: £8,995
Vauxhall has managed to squeeze one of its full sized Corsas into insurance group two.
Auto Express says the 1.0-litre ecoFlex model has the lowest insurance costs of any mainstream supermini.
The eye-catching design, solid interior and big-car features make it an appealing buy for drivers.
Price new: £9,575
The Smart car caused a bit of a stir when it first appeared thanks to its dinky dimensions and bold two-seat layout, making it very useful for driving about town and absurdly easy to park.
The Smart ForTwo is the latest incarnation, which keeps to the same formula; it still has only two seats and is one of the smallest cars on the road measuring just 2.5 metres long,
The entry level 0.8- and 1.0-litre models qualify for group two insurance.
Price new: £11,810
The Peugeot Partner Tepee 1.6 VTi is the passenger version of the Partner van and according to Auto Express easily the most spacious new car that currently qualifies for a group two insurance rating.
It's got a boxy shape that provides plenty of room and has sliding doors which make it a very practical and flexible family car.
If you're after something smaller the Peugeot Bipper Tepee, which is the smallest of Peugeot's family of Tepee MPVs, also gets a group two insurance rating.