Changes to the MoT test, which come into force soon, are aimed at catching drivers who try to dodge speed cameras by using reflective paint or alter their number plates background to make it difficult for cameras to recognise their registration number.
The Department for Transport (DOT) estimates that there could be as many as 33,000 motorists who could be caught out by the new law.
A DOT spokesman said: "The MOT test already includes a basic check of number plates – primarily to ensure that characters are in the correct font and recognisable, and also to ensure that the number plates are not likely to fall off the vehicle.
"The main purpose of this modification to the MOT test is to detect number plates which have backgrounds that are illegal and which prevent identification of the plate".
Other forms of tampering with number plates are already illegal, such as using a screw or tape altering the appearance of a letter or number.
It is now the onus of number plate makers to ensure they only supply legal number plates.
A survey on 1,000 motorists by one insurance company found that more than one in 10 motorists had either tampered with their number plates or would consider doing so.
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