The vast majority of new diesel cars breach pollution limits when assessed on roads instead of test tracks, a new report suggests.

Nine out of 10 diesel cars break the limits, with the average diesel emitting seven times more NOx than is permitted.

One model even breached the limit by a factor of 22, the report found.

Since September 1 new diesel cars sold in the EU must meet a new “Euro 6” emissions target. But it has been suggested by the report that in order to get their models past the regulations, carmakers have utilised a variety of techniques.

These allegedly include using special lubricants, over-inflating tyres, making vehicles lighter and using ultra-smooth test-tracks.

The report by Transport & Environment (T&E) was compiled from recent data. Its authors state the findings show diesel cars emit much greater levels of NOx gases when tested in normal conditions.

T&E highlights the effects on health and the economy due to polluted air, stating it causes half a million premature deaths each year in Europe, as well as 100 million lost working days.

The report also found that car makers sell less polluting models in the USA, where pollution limits are stricter than in Europe.

Greg Archer, T&E's clean vehicles manager, said: "Every new diesel car should now be clean but just one in 10 actually is. This is the main cause of the air pollution crisis affecting cities. Carmakers sell clean diesels in the US and testing should require manufacturers to sell them in Europe too."

Air pollution in the UK has been in breach of EU limits for years, prompting the UK's Supreme Court to order the government to act.

A range of proposals have been put forward to reduce air pollution, including more low-emissions zones and better public transport.

By 2020 the majority of London’s diesel drivers will have to pay £12.50 for the Congestion Charge.