Volkswagen's admission that it falsified emission tests in the US with hidden software has led to widespread speculation that other car manufacturers may be deceiving regulators. Earlier in September, the International Council on Clean Transportation (ICCT) released details of a study that analysed emissions from 32 vehicles that supposedly met the latest European standards, known as Euro 6.

The cars exceeded the Euro 6 limit on NOx emissions (NOx refers to polluting compounds of nitrogen and oxygen) by multiples that averaged between 2.3 and eight, depending on the technologies employed.

Meanwhile, specialist emissions testing company Emissions Analytics has tested more than 200 supposedly Euro 6-complaint vehicles and has found only five that can meet the standard when tested on the road, as opposed to the laboratory. So is all this proof that manufacturers are routinely cheating in testing?

Emissions Analytics CEO Nick Molden does not believe so. He said: "In the US they found VW was 30 or 40 times over the limit on NOx. The worst we've seen is 20 times over. We put that down to early Euro 6 engines with technology that hasn't been properly configured yet, rather than cheating."

Even so, Molden says deliberate manipulation cannot be ruled out and adds that not all industry experts share his view. "There are others, like the campaign group Transport & Environment, who do believe defeat devices are being used," he said, according to the Telegraph.

Testing is set to become more stringent in Europe soon. To date, a key problem has been that vehicles are tested in the laboratory but not on the road. New, more stringent testing planned by the European Commission from 2017 will cover both. As well as laboratory procedures, it will include a Real Driving Emissions test on the road, which is aimed to highlight discrepancies between laboratory and real-world performance.

Meanwhile, a German magazine report has claimed the BMW X3 diesel model emits NOx pollutants up to 11 times higher than European norms and suggested other BMW vehicles were also in breach of the emissions limit.