The use of new police cameras to read car number plates means drivers and enforcement agencies must tighten up their act, a motoring body claims.
ANPR cameras scan the registration plate of a moving vehicle
The AA Motoring Trust said automatic number plate recognition (ANPR) cameras cannot read plates obscured by dirt or with incorrect character spacing.
It said a man with a customised plate was stopped in north Wales because officers did not think it was genuine.
A police spokesman said number plate defects could trigger a spot check.
Police forces across the UK are making increasing use of ANPR cameras, which scan a number plate and then search a database of more than 35 million registrations in just four seconds.
If the registration matches one of the estimated one million "wanted" vehicles on the system, an alert is sounded to the roadside police patrol.
A pilot project by 23 police forces in England and Wales between June 2003 and June 2004 saw more than 180,000 vehicles stopped and more than 3,300 arrests for driving offences.
Also, 13,499 people quizzed at the roadside were later arrested for non-driving offences such as drugs and burglary.
The AA Motoring Trust has claimed the cameras could also see law-abiding motorists mistaken for criminals.
It said innocent motorists could be pulled over if the camera system cannot read a number plate due to it being dirty or having an incorrect spacing of its letters or numbers.
Police say the cameras are needed to spot illegal vehicles and drivers
The trust said a driver in north Wales who had customised a plate he had bought in Northern Ireland was recently pulled over by an ANPR team who at first refused to accept the registration, which spelt the man's name, was genuine.
The trust claims vehicle owners may also find themselves being stopped because their registration had been cloned by criminals and used to evade prosecution for speeding, illegal parking, insurance and other offences.
Paul Watters, head of roads and transport policy for the trust, said drivers needed to be aware that prosecution was now more likely if they failed to keep their number plates street legal.
He said: "Whereas, in the past, police officers may have unofficially turned a blind eye to number plates covered in mud or customised registrations unless they needed to stop a car, ANPR may mean that the days of the fancy or dirty plate are numbered."
Mr Watters said the accuracy and up-keep of data held by the Driver and Vehicle Licensing Agency (DVLA) in Swansea would be "critical" to ensure enforcement based on ANPR.
North Wales Police Inspector Alan Brown said officers using an ANPR system could not know what defect in a registration place had prompted an alert until the vehicle had been stopped.
He said: "If the size, type and spacing of the lettering on the vehicle number plate are not in accordance with traffic legislation, then our ANPR computer will be triggered and officers will stop the vehicle.
"Obviously, once officers have stopped the vehicle, they have a range of options, from a verbal warning through to reporting for summons."