London - Toxic emissions from diesel cars are much worse when temperatures fall below 18 degrees.
That means driving a diesel in Britain - or on the Gauteng Highveld on a winter morning - is a greater threat to human health and the environment.
The problem is caused by engine management systems installed by manufacturers that partially switch off emission controls when it gets colder.
Industry critics have accused carmakers of taking advantage of this because it protects the engine against extreme temperatures and reduces fuel-consumption - but it also leads to higher emissions.
Governments have used the tax system to encourage families to use diesel cars, saying they emit less carbon dioxide - the main greenhouse gas - than petrol engines.
But diesels have caused much higher emissions of other toxic pollutants such as nitrogen oxides. NOx levels in Britain are so far above EU safe limits that officials are threatening to prosecute offenders.
These new claims were made by the testing firm Emissions Analytics, which looked at 213 models from 31 manufacturers. It found a particular issue with diesel cars that comply with emissions standards that came into effect in 2011, known as Euro 5.
Tests showed that on average these vehicles were 3.6 times over the legal limit for poisonous emissions when the ambient temperature was above 18 degrees - but this rose to 4.6 times over the limit when the air temperature dropped.
Chief executive Nick Molden said it was surprising the threshold used to partly turn off emission controls was as high as 18 degrees, which is not extreme.
Millions of diesel cars on British roads could be operating for most of the year with emission controls partly or totally switched off, he said. “The suspicion is, to give the car better fuel economy.”
The latest generation of Euro 6 cars, on sale from September 2015, fared better in the company’s tests. They averaged 2.9 times the limit when the was above 18 degrees, rising to 4.2 at lower temperatures. The higher figure, Molden said, was skewed by three especially bad performers, which he didn’t identify.
Professor Ricardo Martinez-Botas from Imperial College London, the independent engineer overseeing the British tests, said he was ‘shocked’ at higher pollution levels on the road compared with in the lab.
He is calling for car makers to be more open about what they do with temperature.
The Department for Transport 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.”
The Society of Motor Manufacturers and Traders said all vehicles on the road met current legal standards, and tighter emissions rules coming into effect between 2017 and 2020 would remove the temperature issue.