Millions of motorists risk their cars failing the MOT under new rules designed to crack down on illegal number plates, which can't be read by speed or police cameras.
Drivers who spray reflective backgrounds on their number plates in a bid to evade detection by speed cameras also face being failed.
MOT testers will only pass cars with number plates that are not overprinted or shadowed with text, have black characters on white backgrounds, do not have a honeycomb background and do not have a non-reflective border wider than permitted or positioned too close to the characters.
It is believed up to 40,000 cars will face a re-test in the first year of the new scheme.
In a separate move, drivers with personalised number plates who illegally rearrange numbers or letters will be fined £60 rather than £30.
Speed cameras and automatic number plate recognition cameras cannot read the altered number plates - allowing offenders to avoid penalties, says the Government.
The new laws come amid a boom for personalised number plates, from which the Driver and Vehicle Licensing Agency (DVLA) benefits.
The Department for Transport will be putting up the cost of the MOT from £53.10 to a maximum of £54 as it launches a consultation on two new MOT measures: to crack down on dodgy number plates and on unroadworthy towing devices.
A new survey by the leading High Street car insurance retailer, Swinton, calculates that up to 12 per cent of motorists have illegal number plates - that's up to four million drivers.
Nearly 68 per cent of motorists did not know if their number plate conformed to DVLA regulations, and one in five (21 per cent) believed their number plates might not be up to standard.
Steve Chelton, insurer development manager for Swinton, said: 'Many drivers are unaware of the strict guidelines imposed by the DVLA on number plates.' Some drivers have been illegally moving the space between the digits and the letters.
Others have put black caps on the yellow bolts securing the plates, to make figures 1 and 1 look like an H.
Among other changes, some drivers put a black-headed bolt halfway through the letter C to make it look like the letter E.
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