Abstract
The article provides an insight into the constitutional role of customary international law within the EU legal order by focusing on the CJEU’s relevant jurisprudence. It is divided in three parts. First, it examines into the interconnectivity between EU and customary international law and its applicability to EU external action. The second part explores the ways in which international custom can be invoked by private parties in order to review the legality of EU secondary law and, therefore, escape their EU law obligations. In light of the CJEU’s recent controversial judgment in the Emissions Trading Scheme,22 the third part of this article discusses the new conditions under which private parties must rely on customary international law to invoke it before the courts of the Member States to challenge the application of EU regulations to non-European airlines’ activity outside the EU. By way of conclusion, this article will make some observations on the place of customary international law in the EU legal order as an effective standard of review of EU law and a useful tool to curtail EU external competence. This is significant in determining, for instance, the extraterritorial jurisdiction of the EU vis-à-vis the enactment and enforcement of EU legislation to tackle global problems.