In the study by campaign group Transport & Environment, some cars were found to pollute 15 times more than the Euro 6 emissions limits for harmful nitrogen oxides (NOx) allow out on the road.
It comes as new cars are set to be required to pass tough real-world driving emissions tests before gaining approval from the EU from September 1 next year.
Under the new regulations, a portable emissions system will be used to test vehicles for NOx and pollutants on the road, replacing current tests that are carried out in the laboratory.
According to the research, all manufacturers are having difficulty achieving Euro 6 emissions targets in real-world conditions compared to lab tests.
The findings show that diesel models of the Fiat 500X emit 15 times the current legal NOx limits when tested outside the laboratory.
Meanwhile, the Volkswagen Polo 1.4 TDI was found to pollute double what the current regulations allow – the lowest result recorded of all the 26 manufacturers tested by Transport & Environment.
This is despite the fact that VW was last year found to be using software that switched vehicles’ engines to a cleaner mode when emissions were being tested.
Earlier this year, research carried out by the consumer group Which? found that 19 out of 20 diesel cars emitted limit-exceeding amounts of nitrogen oxide.
The Euro 6 emissions standards were introduced in 2014, requiring diesel vehicles to meet strict targets aimed at clamping down on emissions of NOx, which can badly damage people’s health. Find out more about the Euro 6 standard.
They were a step forward from the Euro 5 regulations introduced in 2011, which targeted exhaust particulates and saw many new diesels fitted with special filters.
Car manufacturers were recently granted leeway to produce vehicles that emit up to 2.1 times the current Euro 6 limit in real-world tests between 2017 and 2019.
This is to allow them to adjust to the new style of testing, with the limit brought down to 1.5 times more than the current limit from 2019 and 2021.