04 Nov Systemic Integration After the ICJ’s Right to Strike Advisory Opinion: What It Means for Trade–Labour Disputes
[José Emmanuel González Carbajal is an international lawyer specialising in international economic law, treaty interpretation, and international dispute settlement.]
Trade–labour disputes sit at an uneasy intersection: adjudicators must engage seriously with international labour law, yet they operate under trade agreements with their own carefully negotiated limits. On 21 May 2026, the International Court of Justice (ICJ) delivered its Advisory Opinion on the Right to Strike under ILO Convention No. 87. The Opinion has rightly attracted attention not only for confirming that Convention No. 87 protects the right to strike, but also for what it says about treaty interpretation (paras 66–74, 90–96, 139–142). In particular, it raises an important question for trade–labour adjudication: under what conditions may panels rely on rules of international labour and human-rights law when interpreting labour commitments in trade agreements?
The question is particularly relevant where trade agreements refer to fundamental labour rights, such as freedom of association and collective bargaining, whose substantive content is also addressed in ILO conventions and other international instruments. Article 31(3)(c) of the Vienna Convention on the Law of Treaties (VCLT) requires interpreters to take into account, together with the context, any relevant rules of international law applicable in the relations between the parties. Yet the meaning of this requirement has long been debated, particularly where not all parties to the treaty under interpretation are bound by the external rules invoked.
One restrictive interpretation requires those rules to be binding on all parties to the treaty under interpretation. In EC – Approval and Marketing of Biotech Products, the WTO Panel adopted this reading, interpreting the phrase “applicable in the relations between the parties” as referring to all WTO Members, not merely the disputing parties (para 7.68). A similar interpretation was subsequently followed in Eskosol v. Italy, where the arbitral tribunal held that the relevant rules must apply among all parties to the Energy Charter Treaty (para 125). As Richard Gardiner explains, this restrictive approach presents particular difficulties for multilateral treaties, where participation in other international agreements is rarely identical (Treaty Interpretation, 2nd ed., pp. 311–313). That restrictive reading made Article 31(3)(c) difficult to use in practice, especially for labour and human-rights instruments that do not enjoy universal treaty membership.
However, international legal scholarship had already proposed alternatives to both universal treaty membership and the narrower requirement that only the disputing parties be bound by the external rule. In particular, Professor Joost Pauwelyn argued that the decisive consideration should be whether that rule reflects a common understanding among the parties to the treaty under interpretation (Conflict of Norms in Public International Law, 2003, p. 261). This approach was subsequently reflected in the work of the International Law Commission (ILC) Study Group on Fragmentation (paras 470-472). The WTO Appellate Body also recognised in EC – Large Civil Aircraft the need to balance the relevance of external international obligations with the consistent interpretation of WTO agreements (paras 844–845).
Against this background, the Advisory Opinion is significant because the ICJ expressly endorsed the common understanding approach under Article 31(3)(c). This post argues that the Court’s reasoning offers trade–labour panels an additional interpretative avenue for considering relevant international labour and human-rights rules, even where treaty membership differs. Existing determinations under the EU–Korea FTA and the USMCA Rapid Response Labor Mechanism (RRLM) illustrate how panels have approached the interpretation of labour commitments through other provisions of Articles 31 and 32 of the VCLT. The Advisory Opinion does not replace those approaches. Rather, it clarifies the conditions under which panels may also draw upon Article 31(3)(c), while respecting the obligations and limits established by the applicable trade agreement.
The ICJ Advisory Opinion: Standard, Method, and Controversy
The Court’s central finding on systemic integration appears at paragraph 90 of the Advisory Opinion. The Court held that Article 31(3)(c) does not necessarily require all parties to the treaty under interpretation to be bound by the external rule invoked. Instead, such a rule may be taken into account where it expresses a common understanding among the parties regarding the relevant treaty provision being interpreted. The Court thus confirmed that universal treaty membership is not a sine qua non requirement for applying Article 31(3)(c). On that basis, the Court applied this interpretation when considering Article 8 of the ICESCR and Article 22 of the ICCPR. Although not all parties to Convention No. 87 were parties to both Covenants, the Court examined whether their provisions reflected a common understanding of the protection afforded to freedom of association (paras 91–96). The Court’s approach to treaty interpretation has also been examined in recent commentary, particularly regarding the requirements of Article 31(3)(c) and the role of supplementary means under Article 32 (see Julian Arato and Justina Uriburu), as well as the implications of systemic integration for international human rights law (see Siobhán McInerney-Lankford).
The Court relied on two principal considerations. First, the practice of States not party to one or both Covenants was broadly consistent with recognising the right to strike (para 94). Second, States parties to the Covenants had not entered reservations excluding that right or raised significant objections to its recognition (para 95). Taken together, these elements supported the Court’s conclusion that the relevant rules reflected a common understanding among the parties to Convention No. 87 (para 96).
Nevertheless, the Advisory Opinion leaves open the question of what evidence is sufficient to establish a common understanding under Article 31(3)(c) and the weight to be given to silence, non-objection, or acquiescence (S.O. Nolte, paras 27– 30). This remains particularly relevant for future trade–labour disputes, where the parties may hold different positions on the interpretation of international labour and human-rights obligations.
The Court also drew an important structural distinction between Articles 31 and 32, particularly as regards ILO supervisory materials. To qualify as subsequent practice under Article 31(3)(b), State conduct must establish the agreement of the parties regarding the treaty’s meaning (para 77). In the Court’s view, the practice of ILO supervisory bodies does not, by itself, supply that agreement (para 87). Nevertheless, the Court considered the pronouncements of ILO supervisory bodies as relevant supplementary means of interpretation under Article 32, according considerable weight to their interpretations (paras 116–119).
This distinction matters for trade–labour adjudication. ILO conventions may contain relevant rules of international law capable of being considered under Article 31(3)(c), subject to the requirements of that provision. Pronouncements of ILO supervisory bodies serve a different interpretative function and cannot automatically be treated as binding rules or as subsequent practice establishing the agreement of the parties. The Advisory Opinion therefore reinforces the importance of identifying both the legal character of the external material and the basis on which it is used in treaty interpretation.
Trade-Labour Adjudication: Existing Practice and the Way Forward
The relevance of the Advisory Opinion becomes clearer when considered alongside existing trade–labour adjudication. Panels have already faced questions concerning the relationship between labour commitments under trade agreements and international labour-law instruments. The EU–Korea Panel of Experts and the USMCA RRLM determinations illustrate different ways in which international labour law has informed treaty interpretation through the general rules contained in Articles 31 and 32 of the VCLT.
In EU–Korea, the Panel of Experts operated under an agreement whose labour chapter expressly refers to obligations deriving from ILO membership (paras 61–64). Article 13.4.3 of the EU–Korea FTA commits the parties to respecting, promoting, and realising fundamental rights at work in accordance with those obligations. That textual foundation is important. The Panel interpreted the obligation established by Article 13.4.3 by reference to the ILO Constitution, the 1998 ILO Declaration and principles developed by the Committee on Freedom of Association to elucidate the substantive content of freedom of association (paras 107, 110–118). The relevant ILO instruments were therefore considered in connection with an express treaty commitment, rather than as independent obligations introduced through systemic integration.
The USMCA Rapid Response Labor Mechanism (RRLM) determinations provide a different setting. The mechanism addresses the denial of freedom of association and collective bargaining rights at covered facilities, making the interpretation of the concept of Denial of Rights central to the panels’ analysis. The content of this standard must therefore be assessed in light of the practical conditions under which freedom of association and collective bargaining rights are exercised, restricted, or denied (USMCA, Annex 31–A). In Atento, the Panel interpreted the notion of Denial of Rights in accordance with the text, context, object and purpose of the RRLM with due regard to its practical effectiveness (paras 378, 381), holding that a denial may exist where the effective exercise of freedom of association is inhibited or neutralised in practice, even where formal legal recognition remains (paras 377–384, 432–434). The Panel’s analysis focused on the obligations established by the USMCA and the practical circumstances in which those rights may be denied.
Camino Rojo offers another example of the relevance of international labour-law materials. In an expressly identified obiter dicta section, the Panel referred to ILO Conventions No. 87 and 98, ILO supervisory materials, the Tripartite Declaration of Principles concerning Multinational Enterprises and Social Policy, and the UN Guiding Principles on Business and Human Rights to understand the practical content of that standard (paras 360–365). These instruments provided a broader international framework for considering freedom of association and collective bargaining in the circumstances of the dispute. They were not applied as independent sources of obligations under the USMCA.
Taken together, these determinations illustrate how trade–labour panels have relied on international labour-law instruments and materials to interpret obligations under trade agreements through different approaches under Articles 31 and 32 of the VCLT. Building on this practice, the Advisory Opinion has two implications for future trade–labour adjudication. First, the ICJ’s endorsement of the common understanding approach provides a basis for considering relevant rules of international labour and human-rights law under Article 31(3)(c), even where not all parties to the trade agreement are also parties to the ILO convention or other instrument containing those rules, provided they reflect a common understanding among the parties to the trade agreement regarding the provisions under interpretation. Second, the Court’s interpretation of Convention No. 87 may itself inform the interpretation of trade commitments concerning freedom of association, particularly where those commitments refer to ILO standards. These implications are distinct: the former concerns the conditions for taking external rules into account, while the latter concerns the interpretative relevance of the Court’s findings on the protection of the right to strike.
In practical terms, a panel invoking Article 31(3)(c) would need to identify the relevant external rule, establish its relationship to the labour commitment under interpretation, and assess whether that rule expresses the common understanding required under Article 31(3)(c). The evidence may include the parties’ treaty commitments, reservations, positions and relevant State practice, as illustrated by the Court’s analysis of the ICCPR and ICESCR. The existence of an international labour standard, even if widely recognised, does not by itself establish that the requirements of Article 31(3)(c) are satisfied.
The right to strike provides a concrete example. Where a trade agreement contains a commitment to respect freedom of association by reference to ILO standards, a panel may draw on the ICJ’s interpretation of Convention No. 87 to determine whether that commitment encompasses the protection of strike action. The Court’s finding may serve as persuasive authority on the meaning of the Convention, while the legal basis for considering it must be established under the applicable rules of treaty interpretation. Where the panel invokes Convention No. 87 as a relevant external rule under Article 31(3)(c), it must still establish the required common understanding among the parties to the trade agreement. Where the agreement itself refers to ILO obligations, the Court’s interpretation may instead inform the meaning of that treaty reference under Article 31(1). In either case, the panel must determine the scope of the obligation arising under the trade agreement, rather than assume that the Convention and the trade agreement impose identical obligations.
The significance of this approach lies in providing a further legal basis for considering relevant external rules without requiring identical participation in the underlying treaties. It does not replace interpretation under Article 31(1), particularly where the trade agreement expressly refers to ILO obligations, nor does it alter the supplementary role that supervisory pronouncements may play under Article 32.
Its application must also respect the limits of the treaty under interpretation. The United States–Guatemala case illustrates the point. Under CAFTA-DR, the Panel had to determine not only whether there had been failures to enforce labour law, but whether those failures occurred “in a manner affecting trade” (paras 167–174, 478–485, 504). International labour-law rules may assist in interpreting the labour obligations at issue, but they cannot displace the additional elements expressly required to establish a breach under the applicable agreement.
Systemic integration therefore remains an exercise in treaty interpretation, not a means of introducing obligations that the parties have not undertaken. The relevance of the Advisory Opinion lies in clarifying when external international rules may inform the interpretation of existing labour commitments, while leaving the scope of those commitments and the jurisdiction of adjudicative bodies governed by the trade agreement itself.
Conclusion
The Advisory Opinion confirms an important development in the interpretation of Article 31(3)(c) of the VCLT. It endorses the view that relevant rules of international law need not bind all parties to the treaty under interpretation, provided they express a common understanding among those parties. The Court’s contribution is therefore not to introduce a new approach to systemic integration, but to endorse an existing interpretation of Article 31(3)(c). Nevertheless, the evidence required to establish such a common understanding remains a matter of debate.
For trade–labour adjudication, the approach applied by the ICJ in its Advisory Opinion offers a further interpretative avenue for considering relevant international labour and human-rights rules under Article 31(3)(c). Existing decisions under the EU–Korea FTA and the USMCA RRLM illustrate how panels have drawn on international labour-law instruments and materials through other interpretative routes. The ICJ’s reasoning provides guidance for future disputes in which external rules are invoked despite differences in treaty membership. Panels may take those rules into account where the required common understanding is established, while interpreting the obligations under the trade agreement in good faith, in accordance with its terms, context, and object and purpose. In this respect, the Advisory Opinion strengthens the role of systemic integration in treaty interpretation without altering the source or scope of the obligations being adjudicated.