Drivers are increasingly faced with problems from number plate theft, the AA Motoring Trust has warned.
One in 250 vehicles on London's road may have fake plates
The trust said stiffer controls on plate suppliers has forced criminals wanting to "clone" cars to steal plates from existing vehicles.
Innocent motorists could be saddled with fines and accused of speeding and other offences as a result.
The trust, which said there were 14,176 plate thefts last year, advised car owners to report thefts to the police.
Paul Watters, head of roads and transport for the trust, warned of the dangers of ignoring such thefts.
He said: "You may never know that your car has been cloned until speeding, parking and congestion penalty charge tickets arrive on the doormat.
"You will have great difficulty convincing the authorities it was not your car - they will demand that you prove your innocence."
The trust highlighted the case of a man who commuted into London by train but was hit with bills totalling £8,000 for congestion charge and speeding offences.
The car cloned with his vehicle's number plate had entered the charge zone at least 28 times.
As many as one in 250 vehicles entering the London congestion charge zone may be using false number plates, said the trust.
And more than £14m is lost annually by petrol stations from drive-offs, many involving cloned cars, it added.