UK diesel cars could be belching out high levels of Nitrous Oxides (NOx) pollution throughout much of the year and especially in colder weather, research from Emissions Analytics has shown.
The firm tested 213 models across 31 manufacturers. The company claims that the problem is worst at the Euro 5 level, which became mandatory in 2011.
The research also suggests millions of cars could be on the road with their pollution controls partly turned off.
Nick Molden, chief executive of Emissions Analytics, said it is now up to the government to decide whether they call for a vehicle recall.
He said: â€œThe ambient temperature throughout the UK is nine degrees, so diesel cars could be emitting high levels of NOx pollution throughout much of the year.
â€œOur data shows that for temperatures above 18 degrees, diesel vehicles are 3.6 times over the legal limit for NOx emissions, while for temperatures below 18 degrees the cars were 4.6 times over.
â€œThe companies have not done anything illegal, but it is up to the government to decide whether to tell the manufacturers to recall their cars and update their software.â€
Mr Molden said his â€œconcernâ€ is about the quantity of Euro 5 cars on UK roads.
There are currently 5.1 million Euro 5 diesels on Britainâ€™s roads, and they are likely to be driving around for another 10 to 15 years.
A Department for Transport spokesman said: “The regulations are clear that temperature control devices can be justified to prevent engine damage, but we want to see action to ensure that manufacturers are only using these systems in limited circumstances.â€