Pollution from new diesel cars is six times greater than the official limit, according to “real-world driving” emissions tests that contradict results published by manufacturers.

Some of the latest diesel models produce the same amount of nitrogen oxides (NOx) as older ones, which means they could face the same pollution penalties designed to improve air quality in cities.

Boris Johnson, the mayor of London, last week revealed plans to charge owners of existing diesel cars £10 a day to drive into central London from 2020. The latest diesel models, some of which are already in showrooms, are intended to be exempt from the charge because they have to comply with the new Euro 6 standard for pollutants.

However, a study has found that cars that pass the Euro 6 laboratory test routinely produce much higher levels of NOx when driven on roads under more realistic conditions.

NOx and fine particles from diesel engines cause thousands of premature deaths a year in Britain and trigger asthma attacks. The problem has been worsened by the rapid shift to diesel prompted by tax incentives designed to lower carbon emissions.

The Euro 6 limit for NOx is 80 milligrammes per kilometre, less than half the existing limit of 180 mg/km under Euro 5.

Tests commissioned by the Dutch ministry of infrastructure and the environment found that cars that met the official Euro 6 test produced about 500 mg/km during “real-world driving”.

The study, by TNO, a research organisation, found that the gap between the official test and real-world test was greatest on motorways, where it said Euro 6 cars could produce more NOx than Euro 5 ones.

It concludes that a decade of gradual tightening of Euro standards has failed to clean up the NOx in diesel exhausts.

“This experience over the last ten years gives little hope for Euro 6 in the absence of an improved test procedure, or real-driving emission legislation,” the report says.

Richard Smokers, a TNO consultant, said manufacturers were adjusting the engine management systems of their cars to enable them to pass the “very predictable” conditions of the official test conducted in a laboratory.

In real-world driving, with unpredictable braking and acceleration and variations in humidity and air temperature, the lab results could not be replicated.

Dr Smokers said manufacturers were relying on cheap exhaust gas recirculation systems to reduce NOx rather than more expensive but more effective selective catalytic reduction technology (SCR). He said SCR could reduce NOx by 90 per cent but cost £500 per car.

A spokesman for the Society of Motor Manufacturers and Traders said: “It is widely recognised that there are differences between test results and real-world emissions . . . However, the test is a standardised cycle for comparison purposes that cannot and is not intended to represent the variations of the real world. SMMT accepts the need for a more suitable testing process, and is working closely with manufacturers and regulators to implement a solution.”

Meanwhile, Islington council in London has introduced £20 on-the-spot penalties for drivers who refuse to switch off their engine when parked.

Greg Archer, clean vehicles manager of Transport & Environment, which campaigns to reduce pollution from cars, said: “The continuing high levels of air pollution in cities are largely the result of the way carmakers systematically manipulate tests so that their vehicles perform well in the laboratory but far worse on the roads.

“This means that London may be planning to exempt new diesel vehicles that actually perform no better on the road than older ones.”

Mr Archer said manufacturers were lobbying the European Commission to delay until 2021 the introduction of a compulsory real world emissions test.