Thousands of motorists are breaking the law by driving diesel cars without pollution filters, experts have told BBC 5 live Investigates.
The Driver and Vehicle Standards Agency says 1,800 cars have been caught since 2014.
But experts say they believe the number is much higher, claiming the current MOT test, which requires only a visual inspection, is not fit for purpose.
The DVSA says it plans to introduce changes to improve the test next year.
All new diesel cars produced after 2009 have diesel particulate filters designed to reduce pollution levels, but they can sometimes get clogged up and break down, costing thousands of pounds to replace.
Because of the cost involved, some motorists are opting instead to have them removed.
It is illegal for drivers to use a car with the DPF removed, but it is not illegal for garages to take them out.