An adviser to Europe’s highest court said on Thursday that an EU court made legal errors when it ruled in favour of Apple in a €13 billion ($14 billion) tax case. The development could pose a challenge for the iPhone maker.

The tax case against Apple was part of EU antitrust chief Margrethe Vestager’s efforts to crack down on deals between multinationals and EU countries that regulators consider to be unfair state aid. In its 2016 decision, the European Commission claimed that Apple had benefited from two Irish tax rulings for more than two decades, resulting in an artificial reduction of its tax burden to as low as 0.005% in 2014.

The General Court of the European Union upheld Apple’s challenge in 2020, stating that the regulators had not met the legal standard of proving that Apple had received an unfair advantage.

But Advocate General Giovanni Pitruzzella of the EU’s Court of Justice (CJEU) disagreed, suggesting that CJEU judges should overturn the General Court’s decision and send the case back to the lower court. In a non-binding opinion, he said: “The judgment of the General Court on the ‘tax rulings’ issued by Ireland in relation to Apple should be set aside. He also argued that the Court of First Instance had made several errors of law and had failed to “correctly assess the substance and impact of certain methodological errors which, according to the Commission’s decision, tainted the tax rulings”.