In a landmark decision, the European Court of Justice has ruled that national courts cannot act unilaterally but must first engage the consultative process spelled out by EU rules.

The ruling by the EU's highest court, handed down April 2, potentially lifts a cloud over future litigation involving the Spanish airline Vueling, which brought the appeal.

"EU regulations are cast in very strong stone," said Benoît Le Bret, a competition partner in the Brussels office of Gide Loyrette Nouel, which represents Vueling. "This ruling reminds us that EU rules prevail over any decision by national bodies."

The French case at the base of the appeal involved Vueling's labor practices in France in 2007-08, when the Spanish airline established a base of operations at Charles de Gaulle Airport in Roissy, outside Paris.

French labor inspectors asserted that the airline had obtained certificates of Spanish social security coverage for 75 employees working in France. The certificates carried the address of Vueling's headquarters in Spain as the employees' address.

The labor inspectors took Vueling to court for "concealed employment," asserting that the Spanish certificates were a fraudulent attempt to evade payment of French social security charges for the employees.

Vueling countered that the Spanish certificates should be honored under EU rules on free movement of labor designed to prevent double payment of social coverage.

The French criminal court convicted Vueling and invalidated the Spanish certificates in 2010. Vueling appealed the decision, which was upheld by the Paris Court of Appeal in 2012 and by the Court of Cassation, France's highest court, in 2014.

The French social security authorities notified their Spanish counterparts of the case in 2012 and asked Spain to cancel its certificates, consistent with EU rules on establishing dialogue between national authorities.

A Spanish court ruled in 2014 that, although Spain had canceled the certificates, the coverage should remain valid because too much time had elapsed, complicating the calculation of how contributions should be reimbursed.

The European Court of Justice agreed with Vueling's contention that the French authorities violated EU rules by invalidating the Spanish certificates before starting the consultative process.

The court also ruled that, by taking two years, the Spanish authorities did not make their decision within "a reasonable time," as specified under EU rules.

In a joined case also involving Vueling, the EU court ruled that the principle of res judicata should not apply to subsequent cases based on the same facts because the original criminal judgment was reached in violation of EU rules.

So while the European court's ruling leaves the French criminal conviction intact, it limits the legal reach of the original decision, Le Bret said.

Gide fielded a multidisciplinary team for Vueling, with Le Bret and senior associate Diana Calciu on competition law out of Brussels and an employment law team out of Paris led by partner Foulques de Rostolan and including senior counsel Francis Kessler and counsel Yan-Eric Logeais.