Hungary must open its state-owned monopoly on mobile payment systems in the country to other competitors, the EU’s top court held on Nov. 7.

The Court of Justice said that Hungary’s monopoly is a “disproportionate restriction on the freedom to provide services” and it violates EU law. Hungary must comply with the Court’s judgment “without delay” and allow other service providers into the market, the court said.

The Commission could ask the court to impose financial penalties if it concludes...