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Equality of States and Mutual Membership in European Union Law: Contemporary Reflections

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In line with the solutions adopted in other instruments establishing international organizations, Article 4(2) TEU incorporates the doctrine of equality of States into the EU legal order. But the doctrine of equality of States is not the only factor inspiring the Treaty provision. It is generally accepted that the wording of Article 4(2), echoing the definition of formal equality which is contained in the core international human rights instruments—as well as in the constitutions of States—, is also a clear expression of the strict nexus between States and individuals under EU law. Unsurprisingly, the respect of the equality of Member States is also intimately tied to the protection of fundamental values upon which the Union is based—starting from the respect of the rule of law—and thus contributes to the affirmation of the autonomy of EU law, as well as to the establishment of an idea of “mutual membership within the Union”, that is, the idea that unilateral actions of Member States are likely to have an influence on their mutual relations, even in situations where the linkage with EU law is less strict. Against this background, this chapter explores some recent elements of practice attributable to Member States of the Union, showing a growing recourse to a logic of unilateralism, with a view to assessing their implications for a proper enforcement of the equality principle. To do so, the chapter is structured into four sections. After a global assessment of the equality principle in EU law (Sect. 3.1), Sect. 3.2 looks at recent episodes of unilateral infringements of EU law put in place by some EU countries, justified by the need to protect national prerogatives, and creating tensions in terms of equality among Member States before the Treaties. Section 3.3 considers the judicial unilateralism showed recently by the supreme courts of some Member States in re-shaping the interplay between EU law and municipal law, and the influence of such unilateralism on the affirmation of the equality principle. Section 3.4 concludes the chapter, summarizing its main findings.


  • Equality
  • European Union
  • Mutual membership
  • Principle of sincere cooperation
  • Authority of EU law

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  1. 1.

    See, e.g., Article 2(1) UN Charter. Kokott 2011.

  2. 2.

    Rossi 2017, pp. 3–42.

  3. 3.

    In this sense, see, e.g., Casolari 2020, pp. 147–148.

  4. 4.

    CJEU, van Gend & Loos, Judgment, 5 February 1963, Case 26/62, ECLI:EU:C:1963:1.

  5. 5.

    See, e.g., CJEU, Commission v Italian Republic, Judgment, 7 February 1973, Case 39/72, ECLI:EU:C:1973:13, para 24, where the Court of Justice states that: “For a State unilaterally to break, according to its own conception of national interest, the equilibrium between advantages and obligations from its adherence to the Community brings into question the equality of Member States before Community law and creates discriminations at the expense of their nationals, and above all of the nationals of the State itself which places itself outside the Community rules.” In this sense, the equality among Member States contributes to the enforcement of Title III “Equality” of the EU Charter of fundamental rights. On the equality of individuals at the EU level, see Zaccaroni 2021.

  6. 6.

    Broadly speaking, equality is also listed among the basic values of the Union: see Article 2 TEU.

  7. 7.

    Lenaerts 2020.

  8. 8.

    For a general overview on Article 4 TEU, see Klamert 2019, pp. 35–60.

  9. 9.

    CJEU, Achmea, Judgment (Grand Chamber), 6 March 2018, Case C-284/16, ECLI:EU:C:2018:158, para 34.

  10. 10.

    Editorial Comments 2014, p. 7. See also Craig 2020; Casolari 2020, p. 253. See further discussion in Sect. 3.2.

  11. 11.

    More generally on the notion of “EU structural principle” see Azoulai 2018, pp. 31–45.

  12. 12.

    The impact of recent crises on the European integration process has been stressed, among others, by Cannizzaro 2020, p. 16.

  13. 13.

    Bignami 2020.

  14. 14.

    Bortun 2022.

  15. 15.

    Casolari 2019. See also Wouters and Schmitt 2017.

  16. 16.

    Casolari 2019, pp. 61–63; Barnard 2009.

  17. 17.

    On that Clause see Di Federico 2017 and da Cunha Rodrigues 2019.

  18. 18.

    This is the case, for example, of the enhanced cooperation (Article 20 TEU): Peers 2017.

  19. 19.

    As in the case of inter se agreements concluded by Member States to fulfil EU objectives. Similar agreements have played a relevant role in the context of the response to the sovereign debt crisis: Herrmann 2017.

  20. 20.

    Council Decision (EU) 2015/1523 of 14 September 2015 establishing provisional measures in the area of international protection for the benefit of Italy and of Greece [2015] OJ 2015 L239/146; Council Decision (EU) 2015/1601 of 22 September 2015 establishing provisional measures in the area of international protection for the benefit of Italy and Greece [2015] OJ L248/80.

  21. 21.

    CJEU, European Commission v Poland, Hungary and Czech Republic, Judgment (Third Chamber), 2 April 2020, Joined Cases C-715/17, C-718/17 and C-719/17, ECLI:EU:C:2020:257, para 143. On this case, see Tsourdi 2021.

  22. 22.


  23. 23.

    CJEU, De Gezamenlijke Steenkolenmijnen in Limburg, Judgment, 23 February 1961, Case 30/59, ECLI:EU:C:1961:2.

  24. 24.

    CJEU, Slovenia v Croatia, Opinion of AG Pikamäe, Case C-457/18, ECLI:EU:C:2019:1067, para 138. On that doctrine see Azoulai 2011. Cf also De Witte 2017 and Boucon 2014.

  25. 25.

    CJEU, Micheletti, Judgment, 7 July 1992, Case C-369/90, ECLI:EU:C:1992:295, para 10.

  26. 26.

    CJEU, Commission v France, Judgment, 9 December 1997, Case C-265/95, ECLI:EU:C:1997:595, paras 33–35.

  27. 27.

    CJEU, Melchior, Judgment (Third Chamber), 4 February 2015, Case C-647/13, ECLI:EU:C:2015:54, para 21.

  28. 28.

    CJEU, Morgan and Bucher, Judgment (Grand Chamber), 23 October 2007, Joined Cases C-11/06 and C-12/06, ECLI:EU:C:2007:626, para 24.

  29. 29.

    CJEU, Schumacker, Judgment, 14 February 1995, Case C-279/93, ECLI:EU:C:1995:31, paras 21–24.

  30. 30.

    Neframi 2016; Casolari 2020, p. 88.

  31. 31.

    Cf Wolfrum and Kjima 2010; Biondi et al. 2018.

  32. 32.

    CJEU, European Commission v Poland, Hungary and Czech Republic, Opinion of AG Kokott, Joined Cases C-715/17, C-718/17 and C-719/17, ECLI:EU:C:2019:917, para 254.

  33. 33.

    See, inter alia, CJEU, Commission v Poland, Judgment (Grand Chamber), 24 June 2019, Case C-619/18, ECLI:EU:C:2019:531. On this case law see Van Elsuwege and Gremmelprez 2020.

  34. 34.

    Commission v Poland, supra note 33, para 52.

  35. 35.

    For further discussion, with particular regard to the limits concerning the National Identities Clause, see Rossi 2018 and Di Federico 2017, p. 149.

  36. 36.

    Armati 2019.

  37. 37.

    For a detailed exposé, see Adam 2020.

  38. 38.

    Bundesverfassungsgericht [BVerfG], Case No 2 BvR 859/15. An English language translation of the decision is available at On that decision, see Poli 2020.

  39. 39.

    Decision (EU) 2015/774 of the European Central Bank of 4 March 2015 on a secondary markets public sector asset purchase programme [2015] OJ L121/20.

  40. 40.

    CJEU, Weiss, Judgment (Grand Chamber), 11 December 2018, Case C-493/17, ECLI:EU:C:2018:1000.

  41. 41.

    Bundesverfassungsgericht [BVerfG], Case No 2 BvR 859/15, para 119.

  42. 42.

    Ibid., para 157.

  43. 43.

    Vienna Convention on the Law of Treaties, opened for signature 23 May 1969, 1155 UNTS 331, entered into force 27 January 1980, Article 27.

  44. 44.

    Court of Justice of the European Union, “Press release following the judgment of the German Constitutional Court of 5 May 2020”, Press release No. 58/20, 8 May 2020. Available at:

  45. 45.

    CJEU, Costa c E.N.E.L., Judgment, 15 July 1964, Case 6/64, ECLI:EU:C:1964:66, p. 594: “The executive force of Community law cannot vary from one State to another in deference to […] domestic laws, without jeopardizing the attainment of the objectives of the Treaty […] and giving rise to the discrimination prohibited by [the Treaty itself]”.

  46. 46.

    Cf Fabbrini 2015 and Lindeboom 2020.

  47. 47.

    Ministry of Justice, Communication and Promotion Bureau, “The judgment of the German Constitutional Court confirms Poland’s position in the dispute with the European Commission”, 12 May 2020. Available at:

  48. 48.

    Fleming, Shotter, Hopkins (2020) Eastern European states sense opportunity in German court ruling. Financial Times, 10 May 2020. Available at

  49. 49.

    Polish Constitutional Tribunal, Case No K3/21. An English summary of the decision is available at: On that decision, see Editorial Comments 2021.

  50. 50.

    See European Parliament resolution of 21 October 2021 on the rule of law crisis in Poland and the primacy of EU law, P9 TA(2021) 0439. Available at:

  51. 51.

    See the discussion in Krogel 2021.

  52. 52.

    Manzini 2021.

  53. 53.

    Article 50(1) TEU, cf Repasi 2021. The speech given in October 2021 by the Polish Prime Minister during the plenary session in the European Parliament devoted to the future of Europe is clear in this respect: “Over 85% of Polish citizens say clearly: Poland is and remains a member of the European Union. My government and the parliament majority standing behind it, are part of this pro-European majority in Poland.” Cf Statement by Prime Minister Mateusz Morawiecki in the European Parliament, 19 October 2021. Available at:

  54. 54.

    CJEU, Case C-62/14, Gauweiler, ECLI:EU:C:2015:7, Opinion of AG Cruz Villalón.

  55. 55.

    On the spirit of loyalty inspiring the preliminary ruling mechanism, see Opinion 1/09 Creation of a unified patent litigation system EU:C:2011:123, para 84 and Mengozzi 2015.

  56. 56.

    Gauweiler, Opinion of AG Cruz Villalón, supra note 54, paras 65–66. According to several authors, the Taricco saga would represent a clear illustration of a judicial dialogue carried out in the light of the principle of sincere cooperation. See, e.g., Mengozzi 2020.

  57. 57.

    CJEU, Repubblika, Judgment (Grand Chamber), 20 April 2021, Case C-896/19, ECLI:EU:C:2021:311, paras 61–64. The only exception could be represented by a Member State leaving the Union. Even though the ECJ has made it clear that the activation of Article 50 TEU does not exclude the application of EU principles to the State concerned, which continues to be subject to them “until the time of its actual withdrawal from the European Union” (CJEU, RO, Judgment (First Chamber), 19 September 2018 Case C-327/18 PPU, ECLI:EU:C:2018:733, para 45), it is clear that the exiting process is likely to influence the interaction with other Member States.

  58. 58.

    For a general overview of the EU reaction to the rule of law backsliding in Member States, see von Bogdandy et al. 2021.


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Casolari, F. (2023). Equality of States and Mutual Membership in European Union Law: Contemporary Reflections. In: Amoroso, D., Marotti, L., Rossi, P., Spagnolo, A., Zarra, G. (eds) More Equal than Others?. T.M.C. Asser Press, The Hague.

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