The best new diesel cars are as clean as petrol ' while others ten times over legal limit ' according to new data from Emissions Analytics' EQUA Index,

The company, which specialises in measuring tailpipe emissions, says its data shows some diesel cars emit less NOx in the real-world than the Euro 6 regulatory limits, including the first testing under the new Real Driving Emissions (RDE) regulation.

The best of these vehicles ' 20 of which have been awarded an EQUA Aq 'A' or 'A+' rating ' emit less than 80 mg/km, well within the 2.1 conformity factor limit of 168 mg/km.

But the worst-performing 10% of diesels tested since January 2017 emitted more than 911 mg/km.

And the best performance from a Euro 4 diesel scored 402 mg/km ' less than half the worst of the current crop of diesels.

Across the Euro 5 and Euro 6 stages, the best 10% of diesel engines emitted 46% less NOx than the 10% dirtiest petrol vehicles.

Despite these findings, all diesel cars that have achieved an EQUA Aq 'A' or 'A+' rating will be placed in a higher VED band from April.

Under the change in legislation, announced in November, all diesel cars that have not been tested to RDE2 standards will move up a VED band. RDE is the official testing that measures a vehicle's emissions in a 'real world' driving environment.

The second step of RDE, or RDE2, which comes into force from 2020, will ensure than on-road measured emissions are no more than 50% more than the regulatory limit. No vehicles have yet been tested to the RDE2 standard but some manufacturers may decide to certify to this level early.

Nick Molden, CEO and founder of Emissions Analytics, said: 'The most efficient diesel engines continue to improve while some of the worst remain on the market, meaning the confusion for consumers increases rather than diminishes.

'Diesel sales are falling, as a result, and this may lead to cities and governments over-compensating and restricting diesels more than is justified by our evidence.

'The danger is that motorists who bought genuinely clean diesels ' together with the manufacturers who made them ' suffer the consequences of the general confusion and loss of confidence. We should be alert to the consequences for carbon dioxide and particle emissions as the new car market seems to be shifting back towards petrol cars'

The EQUA Index's latest findings come as pressure grows on government and industry to take immediate, targeted action to improve air quality.

Recently, Professor Dame Sally Davies, the Chief Medical Officer for England, called for immediate action to address and avoid negative health impacts of pollution. In her annual report, the Chief Medical Officer recommended that government's national standards for air pollutants should be increasingly stringent, while being transparent and easy for consumers to understand.

Meanwhile, some local and national authorities are considering banning diesels. In Germany, a court recently ruled that cities that exceed air quality limits can ban older diesel cars without seeking authority from the national government. In contrast, the UK's High Court last month ruled that the government's current air quality plan needs further improvement to meet legal requirements.

The EQUA Index, which is cited in the Chief Medical Officer's report, provides consumers with a simple way of understanding a car's on-road NOx emissions. The Index awards every car on sale today a 'A+' to 'H' rating depending on the amount of NOx it produces in the real world, contributing towards a better understanding of the contributing factors of air pollution caused by engine emissions.

EQUA is designed to evaluate the performance of individual passenger cars across the European Union in terms of fuel economy and tailpipe emissions under real-world driving conditions.

It measures regulated pollutants, including CO, CO2, NO, NO2, NOx, total hydrocarbons and particulate matter, using officially certified Portable Emissions Measurement Systems (PEMS).

Operating since 2011, it has carried out PEMS tests on more than 1600 model variants of passenger car in addition to testing heavy goods vehicles, tractors, taxis, vans and buses. It is the unrivalled expert in the field of on-road emissions monitoring.