1. Introduction This report aims to describe the rules regarding loss of nationality within the Member States of the European Union (EU). Two opposite principles govern these rules. On the one hand, diversity. In the famous Declaration No. 2 on Nationality annexed to the Maastricht Treaty, Member States made clear that “the question whether an individual possesses the nationality of a Member State shall be settled solely by reference to the national law of the Member State concerned.” On the other hand, unity. Nationality laws of the Member States converge towards a common status: European citizenship. That nationality laws in the EU is fragmented is, thus, only a prima facie statement. Together, the legislations of EU Member States are what constitutes the European rules governing loss of European citizenship. Legislation in the EU Member States is also bound, outside the treatise of the EU, by a common international and European legal framework. In 1930, the Convention on Certain Questions Relating to the Conflict of Nationality Laws laid down the foundation of the international regime of nationality law: “It is for each State to determine under its own law who are its nationals. This law shall be recognised by other States in so far as it is consistent with international conventions, international custom, and the principles of law generally recognised with regard to nationality” (Art.15). This provision perfectly exemplifies not only the principle of sovereignty – which gives each State discretionary power to establish its laws on nationality – but also affirms the legitimacy (if not the primacy) of international law to settle the frontiers and limitations of this discretionary power. In this respect, the 1948 Universal Declaration of Human Rights (UDHR) establishes the strongest principle: “No one shall be arbitrarily deprived of his nationality” (Art. 13). In 1948, international limitations on nationality revocation were urgently needed after the trauma of the Soviet and Nazi deprivations of the 1920s and 30s. It took some time, however, to transform this moral principle into a legal obligation. The legal and political programme set by the 1948 UDHR prohibiting ‘arbitrary’ deprivation has now been widely implemented. Two leading treaties1 establish a general constraint on State policies regarding nationality law. First, the 1961 Convention on the Reduction of Statelessness is the first international treaty whose aim is to limit, inter alia, the power of States regarding nationality deprivation, on the one hand, by reducing and constraining the different deprivation measures leading to statelessness (mainly articles 7 and 8) and, on the other hand, by generally forbidding revocation of nationality “on racial, ethnic, religious or political grounds” (article 9). This Convention was ratified by 19 + 1 Member States.2 Second and more recently, the 1997 European Convention on Nationality imposed for the firsttime general constraints on the grounds for nationality revocation, regardless of statelessness prevention. This Convention was ratified by only 13 Member States3 – an indication that the treaty sets a high standard of protection that many Member States are still not willing to reach.4 The Court of Justice of the EU and the European Court of Human Rights both use these two instruments to explore and control nationality legislation and individual measures among Member States. The Court of Luxembourg notably assesses both the legitimacy of the grounds for revocation and the proportionality of the measure when the loss of European citizenship is at stake (see below 3.2.2. for the Tjebbes case, and 3.2.4. for the Rottmann case). The Court of Strasbourg, for its part, examines both the arbitrary character and the 1 For an exhaustive list, de Groot, G. R. and Vonk, O. W. (2016). International Standards on Nationality Law. Texts, Cases and Materials. Oisterwijk: Wolf Legal Publishers. 2 Austria, Belgium, Bulgaria, Croatia, Czechia, Denmark, Finland, Germany, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Netherlands, Portugal, Romania, Slovakia, Spain, and the United Kingdom. France has signed without subsequent ratification. Cyprus, Estonia, Greece, Malta, Poland, Slovenia, and Sweden have neither signed nor ratified. 3 Austria, Bulgaria, Czechia, Denmark, Finland, Germany, Hungary, Luxembourg, Netherlands, Portugal, Romania, Slovakia, and Sweden. 4 Croatia, France, Greece, Italy, Latvia, Malta, and Poland have signed the Convention without subsequent ratification. Belgium, Cyprus, Estonia, Ireland, Lithuania, Slovenia, Spain, and the United Kingdom have neither signed nor ratified. 1