The emissions tests used to test Britain's cars are outdated, inadequate and are full of loopholes, according to some of the industry's leading experts.
The UK's leading independent testing facility, Emissions Analytics, told Sky News that the European test was "not fit for purpose" and that one prestige car, freely available for sale in Britain, had produced a NOx emission reading that was nearly 22 times higher than the European standard.
John German, the scientist who first exposed VW's use of so-called "defeat devices" , said manufacturers were "cheating and using loopholes" in "a race to the bottom".
For a quarter of a century, cars have had to pass a test to show that their emissions do not exceed target scores for harmful substances, such as carbon dioxide and nitrogen oxide.
However, the same cars often produce wildly different results when their emissions are tested during a real drive, rather than in the laboratory.
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And it is a significant topic for everyone, because what comes out of the back of a car affects our health. In particular, NOx gases - nitric oxide and nitrogen dioxide - have been blamed for air pollution and health problems.
It was a topic that burst into the public arena in 2015, when an American study concluded that many cars failed to meet emissions targets and that Volkswagen (IOB: 0P6N.IL - news) had programmed its cars to produce one set of results when they were tested in a laboratory - and quite another when they were actually driven.
The man behind that study was Mr German, from the International Council on Clean Transportation, an American organisation.
In an exclusive interview, he told Sky News that, despite the global furore that followed his revelations, "there hasn't been much of a change of behaviour".
"Government agencies aren't enforcing these emissions rules. We are doing a lot of testing in Europe, India, Brazil, China and we're seeing very high levels of NOx everywhere.
"In Europe there is a structural problem because standards are set by the European Commission centrally but enforcement is done at country level.
"The manufacturers go shopping for which country is going to do the least enforcement. The system ends up being gamed."
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In a workshop in Buckinghamshire, a small group of mechanics are trying to address the balance. It has measured the emissions output of hundreds of cars, all of which have passed European emissions tests and are on sale in this country.
Of more than 130 diesel models that have been tested, only 15 actually repeated that performance when replicated in "real world" driving.
"The European test is not fit for purpose," said Jane Thomas, the company's business manager.
"It hasn't been updated for 20 years. We have driven cars that have met the standard, but then emit 20 times more NOx when we've driven them on the road.
"There are some really clean diesels, and some very dirty diesels, and these are all brand new cars. We can see from the tests that manufacturers can meet these rules when it comes to real-world driving. It can be done, but not everyone is doing it."
Of the 15 diesel cars that have passed the company's analysis, six are Volkswagens, and 10 are made by the Volkswagen group. All 15 are made by German-owned companies. However, a German car also took the dubious honour of recording the highest NOx emissions score of any car on sale at the moment - an Audi A8 that emitted nearly 22 times the standard for NOx gases.
By coincidence, Audi (IOB: 0FG8.IL - news) also had two of the best performing cars - the A2 and A5 - and also scored the best average NOx emission of all manufacturers. This is a complicated landscape.
And it's not so very long since Gordon Brown's government offered tax incentives to persuade drivers to buy diesel cars, believing that a move away from petrol was the key to cutting CO2 emissions.
"The car manufacturers have been seriously misleading us by using the test system to get past the European standards," said Sir David King, who was chief scientific adviser to Mr Brown.
"Frankly (Other OTC: FLKKF - news) , NOx levels in our cities are dangerous to human health and this has to be put at the door of car manufacturers. This is a global problem and it has taken a long while for us to realise what the manufacturers were doing."
Later this year, though, the picture may change. A new emissions test will be brought in, including a section where emissions will measured as a car is driven under "real-world" conditions.
It's described by the Society of Motor Manufacturers and Traders as "the world's toughest testing standard".
John German says he expects the test to "cut loopholes in half" - to address, if not solve, the problem of unreliable emissions testing.
But the bigger picture is of a world that embraces electric cars, alternative fuels, better public transport, car-sharing and autonomous vehicles. And that will be an even bigger challenge than cleaning up diesel.