EU Enlargement Law: History and Recent Developments: Treaty Custom Concubinage?

This note provides a detailed account of the development of the EU enlargement law. Based on the material provided by the latest enlargement round, it outlines the main set of enlargement principles, criteria and procedural tools employed by the Union in the process, also making a sketch of the actual chronology of enlargement events. Based on the analysis of the legal regulation of five rounds of enlargement and making parallels with the notion of customary law as understood in public international law, it argues that the Union enlargements have always enjoyed a dual regulation: by written (mostly Treaty based) and also by customary enlargement law. The existence of customary law explains the consistency of enlargement regulation throughout all the rounds of this process, notwithstanding the stage of the Treaty reform in force at the time of every particular accession. The minimal amendments introduced into the enlargement article by the Treaty Establishing a Constitution for Europe (Art. I-58) suggest that the future enlargements are likely to be building on the body of customary law in force to date. The process of gradual incorporation of customary law into the written law of the EU is also likely to continue.