Thousands of motorists risk their cars failing the MOT under new rules designed to crack down on illegal personalised number plates which can't be read by speed or police cameras.
Drivers who spray reflective backgrounds on their registration plates in a bid to evade detection by speed cameras also face being failed.
MOT testers will only pass cars with cherished number 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.
Up to 40,000 cars will face a re-test in the first year of the new scheme.
And in a separate move, drivers with private number plates who illegally rearrange numbers or letters will be fined £60 rather than £30.
Speed cameras and automatic private number plate recognition cameras cannot read the altered registration plates - allowing offenders to avoid penalties, says the Government.
The new laws come amid a boom for cherished number, 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 cherished 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 personalised number plates - that's up to four million drivers.
Nearly 68 per cent of motorists did not know if their private number plate conformed to DVLA regulations, and one in five believed their plates might not be up to standard.
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.