The consumer watchdog Which claims the majority of cars contravened emissions regulations when it carried out realistic tests, and is calling for more rigorous assessment.

All of the vehicles officially complied with recent EU emission regulations (either Euro 5 or the tougher Euro 6 depending on the age of car) but it said it found 95% of diesel cars, and 10% of petrol cars, pump out more NOx than limits allow.

The majority of petrol cars it tested also exceeded EU carbon monoxide (CO) limits, and even some hybrid cars exceeded NOx and CO limits

It added that some modern cars are so excessive in their production of NOx and CO that, in its tests, they would fail to meet any EU emission limits from this century, and 38 of the cars tested were even unable to meet the Euro 1 emissions standards from 1993.

Which said the New European Driving Cycle (NEDC) test, which is currently used to test emissions, is too lenient and exploitable, and called for more challenging and realistic testing.

In a statement, the Society of Motor Manufacturers and Traders (SMMT) commented: “All cars sold in the UK undergo an official test under EU law to ensure they meet the latest emission standards. The testing method employed by Which is very different from the official EU test, so it is no surprise it delivers different results.

“The existing test is a laboratory test and was never designed to reflect the infinite variations of ‘real world’ driving. New technology now means the test can be changed to make it more representative of what consumers experience. However, given the huge differences in temperature, road and vehicle conditions as well as driving styles, no test can ever replicate exactly what happens on the road all the time.

“Industry wants this new test in place as soon as possible but it must be robust, credible and repeatable – the benchmark of any objective assessment.”