A recent study by testing company Emissions Analytics discovered that emissions of polluting gases from diesel cars are significantly higher when the air temperature is 18C degrees or lower.
The study examined 213 models from 31 manufacturers and highlighted that the problem is strongest with cars approved between 2009 and 2011; so called "Euro 5" cars.
The findings indicate that many millions of diesel cars will be driving around with their pollution controls partially off for more of the time, creating a lot more air pollution than previously realised.
Car manufacturers say that the practice is legal as their vehicles are designed this way to help prevent them from breaking down, helping to minimise repair costs for owners.
European rules do allow manufacturers to cut back on their vehicles' pollution controls as long as it is to protect the engine.
The chief executive of Emissions Analytics, Nick Molden, commented: "I would say from the Euro 5 generation of cars, it's very widespread, from our data. Below 18 degrees many have higher emissions. The suspicion is, to give the car better fuel economy.
"If we were talking about higher emissions below zero, that would be more understandable and there are reasons why the engine needs to be protected. But what we've got is this odd situation where the temperature threshold has been set far too high, and that is a surprise".