The number of cars failing MOT emissions tests has more than doubled since tougher rules were introduced to improve air quality and make roads safer.
Nearly 750,000 cars failed the emissions test in the six months after the change on May 20, compared with 350,000 during the same period last year.
The figures released by the Driver and Vehicle Standards Agency (DVSA) also show the number of diesel vans failing the emissions test has risen from 3,585 to 19,648 year-on-year.
The updated MOT means diesel cars are subjected to tighter smoke limits, while new fail categories state when a vehicle should not be driven until a dangerous defect is repaired.
A number of components and functions are being tested for the first time under the refreshed regulations.
Vehicles that fail the MOT are taken off the road until they are repaired.
Motorists can be fined up to £1,000 for driving a vehicle without a valid MOT.
DVSA chief executive Gareth Llewellyn said: “DVSA’s priority is to help everyone keep their vehicle safe to drive.
“We are committed to making a real difference to those in society whose lives and health are blighted by poor air quality.
“Since introducing the new tighter MOT emissions test in May, nearly 750,000 vehicles have been taken off the road or fixed.”
The changes to how defects are categorised in the MOT - minor, major or dangerous - were designed to make it simpler for motorists to know if their vehicle is safe to drive.
Faults which are deemed “dangerous” or “major” result in the MOT being failed. DVSA advises that a “minor” issue should be repaired as soon as possible.
RAC spokesman Simon Williams said: “While it’s very worrying that so many vehicles have been driving around with dangerous issues, it’s good news for all road users that the new MOT fault categories has identified them for urgent repair as this should lead to fewer accidents.
“There is, of course, cause for concern that some drivers will ignore the test results and continue to drive their vehicles, even if only for a short time before getting them fixed.
“The law is clear that vehicles found to have dangerous faults should not be driven until repaired.
“Overall we feel the introduction of the minor, serious and dangerous fault categories is a positive step that will lead to a reduction in the number of unroadworthy vehicles on our roads.”
There are stricter limits for emissions from diesel cars with a diesel particulate filter (DPF), which captures soot.
Vehicles get a “major” fault if the MOT tester can see smoke of any colour coming from the exhaust or finds evidence that the DPF has been tampered with.
It was feared the new rules could lead to expensive bills for people who have previously taken their cars to unscrupulous garages where DPFs have been removed because they are costly to replace when faulty.
New DPFs often cost more than £1,000, which exceeds the value of many cars on the road.
Among the checks being carried out for the first time under the new regime are:
* If tyres are obviously underinflated
* Fluid leaks posing an environmental risk
* Reverse lights on vehicles first used from September 2009
Motoring groups expressed concern that many drivers were confused about or unaware of the new test.
Vehicles must undergo an MOT on the third anniversary of their registration and every 12 months if they are over three years old.
A number of vehicle parts are checked during MOTs to ensure they meet legal standards, such as lights, seatbelts, tyres and brakes.