According to the consumer group Which?, 95 per cent of diesel cars and 10 per cent of petrol vehicles emit nitrogen oxides (NOx) above the official legal limit.
The group has released a report showing the results of an analysis of the emissions of over 300 cars tested in real-world conditions since 2012.
The Jeep Grand Cherokee was the worst performer out of the 153 diesel cars tested, emitting 15 times the amount allowed, the report concluded.
The report follows the saga surrounding car-maker Volkswagen (VW), which has been embroiled in a scandal for months after it emerged that the firm had installed software in certain diesel models that allowed its cars to pass government emissions tests while emitting 40 times the permitted amount of NOx in real-world driving.
The researchers also found that 65 per cent 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.
Executive director of Which?, 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."
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.”
In addition to VW, Renault has also been dragged into the emissions scandal this week and is currently recalling 15,000 vehicles that are in breach of the regulations.
The French Energy Minister, Segolene Royal, recently said that a number of foreign manufacturers are guilty of exceeding emissions limits, but she has declined to name them.