¹ See de Witte, B., ‘The Future of Variable Geometry in a Post-Brexit European Union’, 24 MJ (2017) p. 153 Google Scholar.

² It is debatable whether loyalty and sincere cooperation are actually synonyms or whether sincere cooperation only covers one dimension of the principle of loyalty. In this article, however, which does not aim at a comprehensive analysis of loyalty, the two terms will be used interchangeably, even though this implies a certain degree of simplification.

³ See, for instance, Wouters, J., ‘Constitutional Limits to Differentiation: The Principle of Equality’, in B. de Witte et al. (eds.), The Many Faces of Differentiation in EU Law (Intersentia 2001) p. 301 Google Scholar; Thym, D., Ungleichzeitigkeit und europäisches Verfassungsrecht (Nomos 2004)Google Scholar; Ott, A., ‘Constitutional Boundaries to Differentiation: How to Reconcile Differentiation with Integration?’, in A. Ott and E. Vos (eds.), Fifty Years of EU Integration: Foundations and Perspective (T.M.C. Asser Press 2009) p. 113 CrossRefGoogle Scholar; Pistoia, E., Limiti all’integrazione differenziata dell’Unione europea [Limits to Differentiated Integration of the European Union] (Cacucci 2018)Google Scholar.

⁴ For an important exception, though only discussing the role of loyalty in constraining international agreements between member states, see Dimopoulos, A., ‘Taming the Conclusion of Inter Se Agreements between EU Member States: The Role of the Duty of Loyalty’, 34(1) Yearbook of European Law (2015) p. 286 Google Scholar.

⁵ This article only addresses loyalty as a constraint on the external action of the member states, whereas it does not consider loyalty-based obligations that bind EU institutions in the context of EU external relations.

⁶ See Stubb, A., ‘A Categorization of Differentiated Integration’, 34(2) Journal of Common Market Studies (1996) p. 283 CrossRefGoogle Scholar; Thym, D., ‘Competing Models for Understanding Differentiated Integration’, in B. de Witte et al. (eds.), Between Flexibility and Disintegration. The Trajectory of Differentiation in EU Law (Edward Elgar 2017) p. 28 CrossRefGoogle Scholar.

⁷ Differentiated integration should therefore be distinguished from flexibility in a wider sense, which encompasses a wide range of derogations from uniformity such as minimum harmonisation or the leeway left to the member states in the implementation of directives. On this distinction, see Tuytschaever, F., Differentiation in European Union Law (Hart 1999) p. 2-3 Google Scholar. For a similar definition, emphasising the link between differentiated integration and the willingness of individual member states to participate in EU policies, see Stubb, supra n. 6, p. 283 (the author defines differentiated integration as ‘a model of integration strategies that try to reconcile heterogeneity within the European Union and different groupings of member states to pursue an array of public policies with different procedural and institutional arrangements’).

⁸ See D. Hanf, ‘Flexibility Clauses in the Founding Treaties, from Rome to Nice’, in de Witte, Hanf and Vos (eds.), supra n. 3, p. 3.

⁹ See Curtin, D., ‘The Constitutional Structure of the Union: A Europe of Bits and Pieces’, 30 CMLR (1993) p. 17 Google Scholar; de Witte, B., ‘The Elusive Unity of the EU Legal Order after Maastricht’, in M. de Visser and A.P. van der Mei (eds.), The Treaty on European Union 1993-2013: Reflections from Maastricht (Intersentia 2013) p. 53 at p. 54Google Scholar.

¹⁰ Protocols were negotiated granting an exemption to the United Kingdom (currently Protocol No. 15) and Denmark (currently Protocol No. 15).

¹¹ A Protocol on Social Policy authorised 11 out of the 12 member states ‘to have recourse to the institutions, procedures and mechanisms of the Treaty’ in order to adopt measures in the field of social policy. In fact, it introduced an opt-out for the United Kingdom. The Protocol, which was supplemented by an Agreement on Social Policy among the 11 participating member states, was repealed by the Treaty of Amsterdam.

¹² This mechanism, originally called ‘closer cooperation’, was renamed ‘enhanced cooperation’ by the Treaty of Nice. Although its essential features have remained unaltered, the rules on enhanced cooperation have also undergone important modifications through Treaty revisions: for an overview, see S. Peers, ‘Enhanced Cooperation: The Cinderella of Differentiated Integration’, in de Witte et al. (eds.), supra n. 6, p. 76.

¹³ Thym, supra n. 6, p. 41.

¹⁴ Peers, supra n. 12, p. 76.

¹⁵ B. de Witte, ‘Variable Geometry and Differentiation as Structural Features of the EU Legal Order’, in de Witte et al. (eds.), supra n. 6, p. 11-15.

¹⁶ On the phenomenon generally, see de Witte, B., ‘Old-Fashioned Flexibility: International Agreements between Member States of the European Union’, in G. de Búrca and J. Scott (eds.), Constitutional Change in the EU - From Uniformity to Flexibility? (Oxford University Press 2000) p. 31 Google Scholar; B. de Witte, ‘Chameleonic Member States: Differentiation by Means of Partial and Parallel International Agreements’, in de Witte et al. (eds.), supra n. 3, p. 231; Heesen, J., Interne Abkommen: völkerrechtliche Verträge zwischen den Mitgliedstaaten der Europäischen Union (Springer 2015)CrossRefGoogle Scholar.

¹⁷ For a similar partition, see Condinanzi, M., ‘L’integrazione differenziata nell’ambito dell’Unione europea’, in M. Vellano (ed.), Il futuro delle organizzazioni internazionali – Prospettive giuridiche [The Future of International Organisations – Legal Perspectives] (Editoriale Scientifica 2015) p. 405 Google Scholar; Pistoia, supra n. 3, p. 13-19. For the purposes of this article, the concept of ‘external differentiation’ does not refer to agreements with third countries aimed to exporting parts of the EU acquis beyond EU borders, a phenomenon which is usually classified as a form of differentiated integration (see, for instance, Ott, supra n. 3, p. 118-120).

¹⁸ See de Witte, B., ‘An undivided Union? Differentiated integration in post-Brexit times’, 55 CMLR (2018) p. 227 at p. 241:Google Scholar ‘Inter se agreements become a true alternative form of variable geometry when they serve the purpose of allowing a group of Member States to move European integration forward in the face of the opposition of other Member States’.

¹⁹ In 1985 five Member States concluded at Schengen an agreement on the gradual abolition of checks at their common borders; other Member States later joined and the agreement was supplemented by an implementing convention in 1990. Both agreements and the acts adopted on their basis, jointly forming the so-called Schengen acquis, were later incorporated into the EU legal framework by the Treaty of Amsterdam.

²⁰ See Dimopoulos, A., ‘The Use of International Law as a Tool for Enhancing Governance in the Eurozone and its Impact on EU Institutional Integrity’, in M. Adams et al. (eds.), The Constitutionalization of Budgetary Constraints (Hart 2014) p. 41 Google Scholar.

²¹ The application of Regulation (EU) No. 1257/2012 of the European Parliament and of the Council of 17 December 2012 implementing enhanced cooperation in the area of the creation of unitary patent protection is made conditional upon the entry into force of the Agreement on a Unified Patent Court (see Art. 18 para. 2 of the Regulation). On the relationship between the two instruments see Baratta, R., ‘The Unified Patent Court – What is the “common” trait about?’, in C. Honorati (ed.), Luci e ombre del nuovo sistema UE di tutela brevettuale [The EU Patent Protection. Lights and Shades of the New System] (Giappichelli 2014) p. 101 Google Scholar; Waldow, C., ‘An Historical Perspective II: The Unitary Patent Package’, in J. Pila and C. Waldow (eds.), The Unitary EU Patent System (Hart 2015) p. 33 Google Scholar. The same condition is set for the entry into force of Council Regulation (EU) No. 1260/2012 of 17 December 2012 implementing enhanced cooperation in the area of the creation of unitary patent protection with regard to the applicable translation arrangements (see Art. 7 para. 2 of the Regulation).

²² Agreement on the Transfer and Mutualisation of Contributions to the Single Resolution Fund. The Single Resolution Fund has been established by Regulation (EU) No. 806/2014 of the European Parliament and of the Council of 15 July 2014 establishing uniform rules and a uniform procedure for the resolution of credit institutions and certain investment firms in the framework of a Single Resolution Mechanism and a Single Resolution Fund and amending Regulation (EU) No. 1093/2010. The purpose of the agreement is to provide the Single Resolution Fund with the financial resources necessary for its functioning. Contributions are collected from credit institutions on a national basis and subsequently transferred to the Single Resolution Fund by the contracting member states. The choice to resort to an intergovernmental agreement has been criticised: see F. Fabbrini, ‘On Banks, Courts and International Law. The Intergovernmental Agreement on the Single Resolution Fund in Context’, 21 MJ (2014) p. 444.

²³ The relevant provisions are Art. 20 TEU and Arts. 326-334 TFEU.

²⁴ Art. 20(2) TEU.

²⁵ Art. 329 TFEU.

²⁶ Art. 326 TFEU.

²⁷ On advantages and disadvantages of external differentiation compared to internal tools of differentiation, see Thym, supra n. 6, p. 49-52; de Witte, supra n. 18, p. 243-244.

²⁸ See Art. 20(1) TEU, which states that enhanced cooperation is established ‘within the framework of the Union’s non-exclusive competences’, by making ‘use of its institutions’ and ‘by applying the relevant provisions of the Treaties’.

²⁹ See second last section infra.

³⁰ See Temple Lang, J., ‘The Development by the Court of Justice of the Duties of Cooperation of National Authorities and Communities Institutions under Article 10 EC’, 31 Fordham Int Law J (2007) p. 1483 Google Scholar; Porchia, O., Principi dell’ordinamento europeo [Principles of the EU Legal Order] (Zanichelli 2008)Google Scholar.

³¹ Gormley, L., ‘Some Further Reflections on the Development of General Principles of Law within Article 10 EC’, in U. Bernitz et al. (eds.), General Principles of EC Law in a Process of Development (Kluwer Law International 2008) p. 303 Google Scholar.

³² Art. 4(3) TEU reads as follows:

‘Pursuant to the principle of sincere cooperation, the Union and the Member States shall, in full mutual respect, assist each other in carrying out tasks which flow from the Treaties.

The Member States shall take any appropriate measure, general or particular, to ensure fulfilment of the obligations arising out of the Treaties or resulting from the acts of the institutions of the Union.

The Member States shall facilitate the achievement of the Union’s tasks and refrain from any measure which could jeopardise the attainment of the Union’s objectives.’

³³ Moreover, Art. 13(2) TEU adds an inter-institutional dimension to sincere cooperation, requiring EU institutions to ‘practice mutual sincere cooperation’.

³⁴ For a detailed and systematic analysis of loyalty in the EU legal order, see Klamert, M., The Principle of Loyalty in EU Law (Oxford University Press 2014)CrossRefGoogle Scholar.

³⁵ ECJ 10 April 1984, Case 14/83, von Colson, para. 26.

³⁶ ECJ 19 November 1991, Joined Cases C-6/90 and C-9/90, Francovich, para. 36.

³⁷ For instance, in its Ruling 1/78 of 14 November 1978, the ECJ held that under Art. 192 of the Euratom Treaty member states were prevented from taking ‘unilateral action […], even if it were collective and concerted action, [which] would have the effect of calling in question certain of the essential functions of the Community and in addition of affecting detrimentally its independent action in external relations’ (para. 33); similarly, in Opinion 2/91 of 19 March 1993 it stated that a ‘duty of cooperation […] results from the requirement of unity in the international representation of the Community’ (para. 36). See also ECJ 14 July 1976, Joined Cases 3, 4 and 6/76, Kramer, para. 42-44; Opinion 1/94 of 15 November 1994, para. 108; Opinion 2/00 of 6 December 2001, para. 18. For a comprehensive overview of the case law on loyalty in the context of external relations, see C. Hillion, ‘Mixity and coherence: The significance of the duty of cooperation’, in C. Hillion and P. Koutrakos (eds.), Mixed Agreements Revisited (Hart 2010) p. 87; E. Neframi, ‘The Duty of Loyalty: Rethinking its Scope through its Application in the Field of EU External Relations’, 47 CMLR (2010) p. 323; F. Casolari, ‘The Principle of Loyal Cooperation: A “Master Key” for EU External Representation?’, in S. Blockmans and R.A. Wessel (eds.), Principles and Practice of EU External Representation, CLEER Working Papers 2012/5; Saluzzo, S., Accordi internazionali degli Stati membri dell’Unione europea e Stati terzi [International Agreements of EU Member States and Third States] (Ledizioni 2018) p. 274-295 Google Scholar; Thies, A., ‘The Search for Effectiveness and the Need for Loyalty in EU External Action’, in M. Cremona (ed.), Structural Principles in EU External Relations Law (Hart 2018)Google Scholar.

³⁸ See Klamert, supra n. 34, p. 234-241, with references to the relevant case law.

³⁹ Art. 20(2) TEU.

⁴⁰ Thym, supra n. 3, p. 247.

⁴¹ Ibid., p. 249.

⁴² See Art. 20(1), second sentence, TEU: ‘Enhanced cooperation shall aim to further the objectives of the Union, protect its interests and reinforce its integration process’.

⁴³ See E. Pistoia, ‘Enhanced cooperation as a tool to… enhance integration? Spain and Italy v Council’, 51 CMLR (2014) p. 247 at p. 258.

⁴⁴ By contrast, neither Protocol No. 22 on the position of Denmark nor the provisions granting the United Kingdom and Denmark an opt-out from the adoption of the common currency provide for selective participation in EU acts covered by the exemption. In such cases, participation of the ‘outs’ can still be secured either through international agreements or through the establishment of special arrangements in secondary law instruments. An interesting example of the latter is provided in Council Regulation (EU) No 1024/2013 of 15 October 2013 conferring specific tasks on the European Central Bank concerning policies relating to the prudential supervision of credit institutions, which under certain conditions allows non-euro area Member States to participate in the new framework for banking supervision centered on the European Central Bank.

⁴⁵ See Art. 5 Protocol No. 19; Art. 3 and, for proposals amending existing acts, Art. 4a Protocol No. 21.

⁴⁶ Art. 5(3) Protocol No. 19; Art. 4a(2) Protocol No. 21.

⁴⁷ See ECJ 8 September 2015, Case C-44/14, Spain v Parliament and Council, para. 47.

⁴⁸ Art. 4 of Protocol No. 21.

⁴⁹ At least not in general terms. A specific limit is to be found in Art. 344 TFEU, which preserves the jurisdiction of the Court of Justice by prohibiting Member States from submitting disputes concerning the interpretation or application of the Treaties to any method of settlement other than those provided for therein.

⁵⁰ See notably de Witte (2000), supra n. 16, p. 39-55; de Witte (2001), supra n. 16, p. 240-245; Heesen, supra n. 16, p. 197-376; Thym, supra n. 6, p. 52-55.

⁵¹ See Klabbers, J., Treaty Conflict and the European Union (Cambridge University Press 2009) p. 209 Google Scholar; Heesen, supra n. 16, p. 233; de Witte, supra n. 18, p. 243.

⁵² ECJ 30 June 1993, Joined Cases C-181/91 and C-248/91, European Parliament v Council of the European Communities, para. 16; ECJ 2 March 1994, Case C-316/91, European Parliament v Council of the European Union, para. 26; ECJ 27 November 2012, Case C-370/12, Pringle, para. 158.

⁵³ It has even been argued that ‘a more severe pre-emption standard’ should apply to joint actions of the Member States as compared to their unilateral action (de Witte (2000), supra n. 16, p. 41-42).

⁵⁴ ECJ 27 February 1962, Case 10/61, Commission v Italy, although specifying that Community law would take precedence ‘in matters governed by the EEC Treaty’ over agreements between Member States ‘concluded before its entry into force’. In the later Matteucci case, the Court held that primacy also applies when the agreement concerns an area not governed by EU law but is liable to interfere with the application of EU measures (ECJ 27 September 1988, Case 235/87, Matteucci, para. 16). See also, in clearer and more general terms, ECJ 15 January 2002, Case C-55/00, Gottardo, para. 33, where the Court stated that ‘when giving effect to commitments assumed under international agreements, be it an agreement between Member States or an agreement between a Member State and one or more non-member countries, Member States are required […] to comply with the obligations that Community law imposes on them’.

⁵⁵ Pringle, supra n. 52, paras. 108-147.

⁵⁶ ECJ 20 September 2016, Joined Cases C-8/15 P to C-10/15 P, Ledra Advertising, paras. 57-59.

⁵⁷ See de Witte, B., Beukers, T., ‘The Court of Justice approves the creation of the European Stability Mechanism outside the EU legal order: Pringle ’, 50 CMLR (2013) p. 805 at p. 829Google Scholar.

⁵⁸ See Dimopoulos, supra n. 4, p. 286.

⁵⁹ ECJ 6 March 2018, Case C-284/16, Achmea, para. 32-37.

⁶⁰ Achmea, supra n. 59, para. 58.

⁶¹ A. Dimopoulos, ‘Achmea: The principle of autonomy and its implications for intra and extra-EU BITs’, EJIL Talk, 27 March 2018, <www.ejiltalk.org/achmea-the-principle-of-autonomy-and-its-implications-for-intra-and-extra-eu-bits/>, visited 8 July 2018.

⁶² Autonomy has been defined as ‘a systemic or constitutional principle derived from the fundamental objectives of the EU legal system’ (M. Klamert, ‘The autonomy of the EU (and of EU law): Through the kaleidoscope’ 42 ELR (2017) p. 815 at p. 816-817).

⁶³ ECJ 15 July 1964, Case 6/64, Costa v ENEL (‘It follows from all these observations that the law stemming from the Treaty, an independent source of law, could not, because of its special and original nature, be overridden by domestic legal provisions, however framed, without being deprived of its character as Community law and without the legal basis of the Community itself being called into question’).

⁶⁴ A close link between autonomy and the ‘essential’ or ‘specific’ characteristics of EU law is established, in particular, in Opinion 2/13 of 18 December 2014 (para. 167 and 174) and in the Achmea judgment, supra n. 59 (para. 33 and 35).

⁶⁵ Matteucci, supra n. 54, para. 19.

⁶⁶ ECJ 3 March 2009, Case C-205/06, Commission v Austria; ECJ 3 March 2009, Case C-249/06, Commission v Sweden; ECJ 19 November 2009, Case C-118/07, Commission v Finland.

⁶⁷ Dimopoulos, supra n. 4, p. 313. Similar clauses are indeed common. For instance, Art. 2(2) of the Treaty on Coordination, Stability and Governance in the Economic and Monetary Union provides that the agreement ‘shall apply insofar as it is compatible with the Treaties on which the European Union is founded and with European Union law. It shall not encroach upon the competence of the Union to act in the area of the economic union’. Although many other inter se agreements include similar provisions, their scope and wording are not always identical. For instance, the priority rule contained in the ESM Treaty is more narrowly phrased and somewhat ambiguous. It provides that memoranda of understanding negotiated on behalf of the ESM ‘shall be fully consistent with the measures of economic policy coordination provided for in the TFEU, in particular with any act of European Union law, including any opinion, warning, recommendation or decision addressed to the ESM Member concerned’.

⁶⁸ Cf S. van den Bogaert and V. Borger, ‘Differentiated Integration in EMU’, in de Witte et al. (eds.), supra n. 6, p. 209 at p. 229-230.

⁶⁹ See, for instance, Opinion 2/13, supra n. 64, para. 173.

⁷⁰ See ECJ 30 May 2006, Case C-459/03, Commission v Ireland (MOX Plant), para. 169.

⁷¹ Achmea, supra n. 59, para. 58.

⁷² See B. Hess, ‘The Fate of Investment Dispute Resolution after the Achmea Decision of the European Court of Justice’, Max Planck Institute Luxembourg for Procedural Law Research Paper Series No. 2018 (3), p. 10; F. Munari and C. Cellerino, ‘EU Law is Alive and Healthy: The Achmea Case and a Happy Good-bye to Intra-EU Bilateral Investment Treaties’, SIDIBlog, 17 April 2018, <www.sidiblog.org/2018/04/17/eu-law-is-alive-and-healthy-the-achmea-case-and-a-happy-good-bye-to-intra-eu-bilateral-investment-treaties/>, accessed 8 July 2018.

⁷³ See ECJ 5 May 1981, Case 804/79, Commission v United Kingdom, para. 28.

⁷⁴ ECJ 2 June 2005, Case C-266/03, Commission v Luxembourg, para. 60; ECJ 14 July 2005, Case C-433/03, Commission v Germany, para. 66.

⁷⁵ ECJ 20 April 2010, Case C-246/07, Commission v Sweden. For a detailed analysis, see M. Cremona, ‘Case C-246/07, Commission v Sweden (PFOS), Judgment of the Court of Justice (Grand Chamber) of 20 April 2010’, 47 CMLR (2010) p. 1639; G. De Baere, ‘“O, Where is Faith? O, Where is Loyalty?” Some Thoughts on the Duty of Loyal Co-operation and the Union’s External Environmental Competences in the Light of the PFOS Case’, 36 ELR (2011) p. 405.

⁷⁶ Commission v Sweden, supra n. 75, para. 103-104.

⁷⁷ Ibid., para. 89.

⁷⁸ Cf Dimopoulos, supra n. 4, p. 304.

⁷⁹ See Craig, P., ‘The Stability, Coordination and Governance Treaty: Principle, Politics and Pragmatism’, 37 ELR (2012) p. 231 Google Scholar at p. 241-242; Dimopoulos, supra n. 20, p. 62.

⁸⁰ In the very first intra-EU investment case decided after the Achmea ruling, the arbitral tribunal opted for a very restrictive interpretation of the judgment, holding that it did not apply to a multilateral agreement such as the Energy Charter Treaty and had therefore ‘no bearing’ upon the dispute (Masdar Solar & Wind Cooperatief U.A. v Kingdom of Spain, ICSID Case No. ARB/14/1, para. 679-683). Although this was not an open rejection of the Court’s ruling, it suggests that international arbitral tribunals may not be the keenest supporters of the primacy and autonomy of EU law, to say the least.

⁸¹ Dimopoulos, supra n. 4, p. 314.

⁸² See A. Delgado Casteleiro and J. Larik, ‘The Duty to Remain Silent: Limitless Loyalty in EU External Relations?’, 36 ELR (2011) p. 524 at p. 533; Casolari, supra n. 37, p. 18.

⁸³ See Constantinesco, V., ‘Les clauses de coopeération reinforcée’, 33 RTDE (1997) p. 751 at p. 755Google Scholar.

⁸⁴ See Fabbrini, F., Economic Governance in Europe. Comparative Paradoxes and Constitutional Challenges (Oxford University Press 2016) p. 197-199 CrossRefGoogle Scholar.

⁸⁵ ‘Member States which wish to establish enhanced cooperation between themselves within the framework of the Union’s non-exclusive competences may make use of its institutions and exercise those competences by applying the relevant provisions of the Treaties […]’ (emphasis added).

⁸⁶ See Ehlermann, C.-D., ‘Différenciation, flexibilité, coopération renforcée: les nouvelles dispositions du traité d’Amsterdam’, RMUE (1998) p. 53 Google Scholar at p. 66; Gaja, G., ‘How Flexible Is Flexibility Under the Amsterdam Treaty?’, 35 CMLR (1998) p. 855 at p. 869Google Scholar.

⁸⁷ Cf Klamert, supra n. 34, p. 295.

⁸⁸ Although the Court of Justice did not address this point, A.G. Kokott argued that the availability of enhanced cooperation as such does not provide a bar to the conclusion of international agreements between member states (View of A.G. Kokott delivered on 26 October 2012, Case C-370/12, Pringle, para. 174).

⁸⁹ ECJ 30 June 1993, Joined Cases C-181/91 and C-248/91, European Parliament v Council of the European Communities and Commission of the European Communities.

⁹⁰ Cf Rossi, L.S., ‘Intégration différenciée au sein et à l’extérieur de l’Union: de nouvelles frontières pour l’Union?’, in G. Amato et al. (eds.), Genèse et destinée de la Constitution européenne (Bruylant, 2007) p. 1219 at p. 1227Google Scholar.

⁹¹ See Pistoia, E., ‘Outsourcing EU Law While Differentiating European Integration: The Unitary Patent’s Identity in the Two “Spanish Rulings” of 5 May 2015’, 41 ELR (2016) p. 711 at p. 716Google Scholar.

⁹² See Gaja, supra n. 86, p. 870; Amtenbrink, F. and Kochenov, D., ‘ Towards a More Flexible Approach to Enhanced Cooperation ’ in A. Ott and E. Vos (eds.), Fifty Years of European Integration: Foundations and Perspectives (TMC Asser Press 2009) p. 181 at p. 183CrossRefGoogle Scholar.

⁹³ For a similar view, albeit inferred from the Pringle judgment, see Rossi, L.S., ‘The Principle of Equality among Member States of the European Union’, in L.S. Rossi and F. Casolari (eds.), The Principle of Equality in EU Law (Springer 2017) p. 3 at p. 23CrossRefGoogle Scholar. An apparently more nuanced position is taken by Thym, who argues that based on loyalty member states could be expected to ‘investigate the feasibility’ of internal differentiation before resorting to international law (Thym, supra n. 6, p. 55).

⁹⁴ Cf Dimopoulos, supra n. 4, p. 314-315.

⁹⁵ Spain and Italy v Council, para. 53.