France “systematically and persistently exceeded the annual limit value for nitrogen dioxide since Jan. 1, 2010,” the court said in a statement.
The European Commission, the EU's executive arm, referred the matter to the European Court of Justice (ECJ) in May 2018 after almost a decade of warnings that went unaddressed.
France is the first of several member states, including Germany and Britain, that the commission has sued in court after it stepped up its anti-pollution fight in the wake of the so-called “Dieselgate” scandal that erupted in 2015.
The motors caught up in the scandal – in which automakers installed special emission-cheating devices into their car engines – are the main emitters of nitrogen oxides that the European Environment Agency says are responsible for 68,000 premature deaths per year in the EU.
Nitrogen dioxide is toxic and can cause significant respiratory problems as one of the main constituents of traffic-jam smog.
Under EU rules, member countries are required to keep the gas to under 40 micrograms per cubic metre — but that level is often exceeded in many traffic-clogged European cities.
The judgement opens the way to possible sanctions at later stage if Paris does nothing to fix the situation.
The case involved 24 areas in France, including the cities of Paris, Lyon, Marseille, Nice and Strasbourg, but also the alpine Arve Valley, France's lorry-clogged gateway to Italy.
While France does not dispute the violation, it argued that the application of European air quality legislation “must be assessed in the light of the structural difficulties encountered” in applying it, the court said in a statement.
But for the judges EU law demands that when high pollution is found, the member state concerned is required to draw up an air-quality plan and to ensure that the danger period is “as short as possible”.
The court said “France clearly did not adopt, in due time, appropriate measures,” noting an “overrun for seven consecutive years”.