As a result, the campaign group gap says the gap “has become a chasm and, without action, will likely grow to 50% on average by 2020".

T&E added that the gap in performance found in many car models has grown so wide that it cannot be explained through known factors including test manipulations and it is calling on EU governments to extend probes into defeat devices to CO2 tests and petrol cars too.

The organisation says its research shows the gap in performance is up to 50% for some models. Mercedes cars have an average gap between test and real-world performance of 48% and their new A, C and E class models have a difference of over 50%. The BMW 5 series and Peugeot 308 are just below 50%.

Greg Archer, clean vehicles manager at T&E, said: “This widening gap casts more doubt on how carmakers trick their customers in Europe to produce much better fuel efficiency in tests than can be achieved on the road. The only solution is a comprehensive investigation into both air pollution and fuel economy tests and all car manufacturers to identify where unfair and illegal practices, like defeat devices, may be in use. There must also be a comprehensive overhaul of the testing system.”