The European Commission in its 2017 ruling ordered the sport's governing body to scrap "disproportionately punitive" penalties on athletes who take part in new money-spinning events.
The EU competition watchdog's investigation followed a complaint by Dutch Olympic speed skaters Mark Tuitert and Niels Kerstholt who said ISU's threats of a lifetime ban put them off from competing in lucrative Ice Derby events run by a South Korean company.
The landmark case has the potential to be as important a milestone as the 1995 court ruling involving Belgian soccer player Jean-Marc Bosman, which paved the way for the free movement of players in the EU.
ISU challenged the EU antitrust decision in 2020 at the Luxembourg-based General Court, Europe's second-highest, but lost.
"The judgment of the General Court which had confirmed the anticompetitive nature of rules of the International Skating Union should be set aside," Advocate General Athanasios Rantos at the EU Court of Justice (CJEU), Europe's highest, said.
He said an entity acting as both the regulator and organiser of sporting events does not in itself breach EU competition laws.
"Sports federations may, under certain conditions, deny market access to third parties, without this constituting an infringement of competition law, provided that the denial of access is justified by legitimate objectives and that the measures taken by those federations are proportionate to those objectives," Rantos said.
He proposed that the case be referred back to the General Court.
The case is C-124/21 P International Skating Union v Commission.
