Companies could have a hard time challenging the constitutionality of France’s proposed digital tax thanks to an opinion by the country’s highest court.

The ruling, released March 6, confirms that the country’s proposal is in line with French law. But it warned that any domestic laws could be superseded by a European Union directive on taxing the digital economy. EU law takes precedence over any member nation’s domestic rules.

The government sought the Conseil d'État’s opinion to prevent a repeat...