The cities of Brussels, Madrid, and Paris will face off against the European Commission on Thursday (17 May), as they seek to annul the EU legislation that eased car emission limits.

The three cities all suffer from high levels of dangerous nitrogen oxides (NOx) emissions, caused in part by diesel vehicles.

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"The citizens of Paris and cities around the world demand clean air to breathe," said Anne Hidalgo, mayor of Paris. (Photo: Joe deSousa)

In front of the Court of Justice of the EU they will contest the new Real Driving Emissions (RDE) test – not because of its increased accuracy in measuring emissions, but because its introduction was accompanied by an easing of permitted levels.

However, ultimately their effort could result in the legal basis for the test being entirely derailed – creating a legal vacuum.

The RDE test was introduced in the wake of the Dieselgate scandal, which saw diesel cars being approved which were 'clean' during the laboratory test, but actually emitted much more pollutants in daily on-road use.

But while the new test measures NOx emissions more accurately, carmakers were given more leeway to pass it.

Under the previous test, national authorities were only allowed to certify cars for the market if their NOx emissions remained below 80 milligrammes per driven kilometre (mg/km).

In October 2015 however, EU member states decided to allow a limit of 168 mg/km until 2020 under the new test – a result of heavy lobbying by the car industry.

"The citizens of Paris and cities around the world demand clean air to breathe," said Anne Hidalgo, mayor of Paris, in a statement published on Tuesday.

"It would be a betrayal of the people of Europe that car manufacturers and industrial lobbies can dictate the rules that regulate some of their most polluting products. … We need the European Union to support us, not give regulatory protection to air pollution," added Hidalgo.

The three capitals' basic argument is legalistic: they argue that the European Commission did not have the legal authority to make the decision.

The procedure used to introduce the RDE test was the comitology method, which mostly excludes the European Parliament.

The original NOx emissions limits were adopted in the so-called 'co-decision procedure' – in which the European Parliament has equal footing with the Council of the EU, where member states meet.

In an internal paper dated 16 June 2015, the commission itself questioned the legality of its move to introduce such a transition period for the car industry.

The note, written by Daniel Calleja, director-general at the commission's internal market and industry department at the time, has been seen by EUobserver.

It said introducing the RDE test in two phases, whereby during the first phase the EU would be less strict, would conflict with the legislation on the so-called Euro 6 emissions standards already in place.

Calleja wrote in 2015 that an "appropriate legal justification" for relaxing the emissions standards during a transition phase "still need[ed] to be developed".

On the other hand, it could also be argued that the parliament did have an opportunity to voice opposition to the new regulation.

In February 2016, MEPs were given the opportunity to veto the bill – which they did not.

Another reason for not annulling the RDE regulation would be that it would thus create a legal vacuum.

The RDE test has been mandatory for carmakers seeking approval to sell their cars in the EU since September 2017.

It is unclear what would happen if the test's legal basis suddenly ceased to exist.