A large number of cars are breaking official emissions limits, according to a study carried out in the wake of the Volkswagen diesel scandal.
Consumer group Which? analysed detailed emissions data from more than 300 cars tested in real world conditions since 2012.
Some 95% of diesel cars and 10% of petrol vehicles were found to emit nitrogen oxides (NOx) above acceptable levels.
The Jeep Grand Cherokee was the worst performer out of the 153 diesel cars tested as it emitted 15 times the amount allowed, the report concluded.
Researchers also found that two-thirds (65%) of the petrol cars tested broke limits on carbon monoxide emissions.
Despite the results of the study, all the vehicles complied with emissions regulations when undergoing official tests.
Tougher testing methods are due to be introduced from September next year but Which? is calling for this to happen sooner.
The organisation's executive director Richard Lloyd said: "Car emissions and fuel claims are important factors when buying a new car, so drivers will be shocked by the results of our testing.
"The current official tests are clearly not fit-for-purpose and we urgently need a new regime putting in place that reflects the reality of how we drive."
Volkswagen Group admitted in September that it fitted software to engines in a bid to cheat emissions tests. Some 11 million diesel vehicles are affected worldwide, including almost 1.2 million in the UK.
A spokesman for the Society of Motor Manufacturers and Traders said: "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."
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