ICJ Advisory Opinion on the Right to Strike Under ILO Convention 87: Reflections on Systemic Integration from the Perspective of International Human Rights Law

[Siobhán McInerney-Lankford is a human rights lawyer who serves as Head of Unit (Equality, Inclusion and Sustainability) and Legal Task Force Chair at the EU Fundamental Rights Agency. The opinions expressed in this piece are solely those of the author and should not be attributed to the EU FRA.]

Introduction

This post considers the ICJ Advisory Opinion on The Right to Strike under ILO Convention No. 87 of 21 May 2026 (hereinafter “the Advisory Opinion”) from the perspective of international human rights law. 

The request for the Advisory Opinion emanated from a dispute within the ILO in which the Employers Group contested the notion that the Freedom of Association and Protection of the Right to Organize Convention 1948 No.87 (hereinafter “Convention No. 87”) encompassed the right to strike. The following question was put to the Court by the ILO Governing Body: “is the right to strike of workers and their organizations protected under the Convention (No. 87)?”

Convention No. 87 does not contain an explicit reference to the right to strike. Adopting a functional and teleological approach, the Court concluded that the ordinary meaning of the relevant terms of the Convention, read in good faith, in their context and in light of the object and purpose of the treaty, indicated that the protection of the right to strike is encompassed in the protection of the freedom of association enshrined in Convention No. 78. 

Article 31(3)(c) and the Right to Strike

The Opinion represents a significant judicial development for the principle of systemic integration derived from Article 31(3)(c) of the Vienna Convention on the Law of Treaties and a number of preliminary points can be recalled here.

The provision states that “any relevant rules of international law applicable in relations between the parties shall be taken into account when interpreting a treaty.” A threshold matter therefore, reflected in the Court’s opinion, as well as in the Separate Opinion of President Iwasawa, is that this is a mandate and not an option. As McLachlan noted in The Principle of Systemic Integration in International Law: “the requirement to take account of other applicable treaty rules is not a matter of caprice or option: it is a legal obligation in the interpretation of the treaty in question” (page 396). At the same time, Article 31(3)(c) requires relevant rules to be taken into account. This means that there is a significant degree of flexibility afforded the interpreting authority in determining how the external rule may inform the interpretation at hand. 

What is a relevant rule of international law applicable in relations between the parties for the purposes of Article 31(3)(c) in relation to the right to strike, assessed from the perspective of international human rights law? The answer to this question turns on the content of States’ obligations to protect, respect and fulfill the right to freedom of association protected under Convention No. 87 as assessed in 2026, and whether such obligations also extend also to the right to strike. 

The ICESCR expressly protects the right to strike (Article 8) and the ICCPR has been interpreted to protect the right to strike through its protection of the right to freedom of association (Article 22(1), also taking account of the opinions of the Human Rights Committee and the Committee on Economic, Social and Cultural Rights. 

A central question in the Court’s opinion was whether the absence of an identity of the parties to both the ILO Convention and the “external treaties” – in this case the ICESCR and the ICCPR – prevented the latter from qualifying as a relevant rule of international law applicable between the parties within the meaning of Article 31(3)(c). 

The Court held that a rule may be generally applicable in the relations between the parties if it expresses their “common understanding” regarding certain provisions of the treaty under interpretation. A key distinction in this regard, noted in the Separate Opinion of President Iwasawa, is that Article 31(3)(c) requires a common understanding, not agreement, such that not all parties to the treaty under interpretation need to be bound by the external treaty. 

The Court adopted a pragmatic and empirical approach to determining common understanding, focusing on there being a “high degree of overlap” in treaty membership. Such a common understanding may be presumed “when rules contained in another treaty have been so widely adopted that they can be considered implicitly accepted by all the parties to the treaty being interpreted.” In this case a high degree of overlap exists between States parties to Convention No. 87 and the ICESCR (173 States parties) – only 8 States parties to Convention No. 87 are not parties to the ICESCR; and the ICCPR (175 states parties) – only 7 States parties to Convention No. 87 are not parties to the ICCPR. Only four States are party to neither the ICCPR nor the ICESCR. “In these circumstances, the Court considers that the two Covenants contain relevant rules that must be taken into account when interpreting Convention No. 87.”

In this author’s opinion, the Court adopted the correct interpretative approach based on the substantive and structural relevance of obligations under the ICESCR and ICCPR to Convention No. 87. On the one hand it rejected a formalistic perspective that would have required an exact identity of the parties which would in turn eviscerate Article 31(3)(c) of its practical import for multilateral treaties by treating them as a bundle of bilateral agreements. On the other hand, it avoided an overly expansive reliance on external treaties to which few of the parties to the treaty being interpreted were party, which would subvert the principle of consent and undermine reciprocity underlying the Vienna Convention on the Law of Treaties (1969). 

The Separate Opinion of Judge Nolte evinces a nuanced approach. While he agrees in principle that relevant rules of international law that are applicable in the relations between parties under Article 31(3)(c) need not formally bind all parties, he cautions against an interpretation of that provision that would dilute the principle of consent underpinning Article 31(3)(c). The separate opinion of Judge Bhandari similarly notes that systemic integration must be applied cautiously and consistently with the principle of state consent. 

Relevant Rules of International Law: The Nature of the Obligation 

A key term in Article 31(3)(c) is “relevant” in determining the manner and extent to which external rules should be taken into account. It is submitted that the focus of such an inquiry should remain on the substantive norm and the content of relevant rights and obligations (McLachlan page 398) . This approach places the norm at the center of the interpretative exercise rather than emphasizing the exactitudes of identical treaty membership. As the International Law Commission opined in 2006 in relation to the principle of harmonization: “It is a generally accepted principle that when several norms bear on a single issue they should, to the extent possible, be interpreted so as to give rise to a single set of compatible obligations.” The determination of relevance must therefore be driven by a concern with ensuring the coherence and integrity of the norm.

Such a focus is reflected in the Court’s approach and weighs against a formalistic outlook on identity of parties. As other commentators have noted, identity of treaty membership is preferable, but it also remains just one factor in determining relevance. Indeed, an insistence on an identity of membership would defeat the purpose of Article 31(3)(c) and potentially result in greater incoherence, uncertainty and fragmentation since it might imply or require a change in interpretation every time a State party joined or left the external treaty regime. 

The norm at issue here was the scope and content of the right to freedom of association understood in 2026: and the correlative obligations of states parties to Convention No. 87 to respect, protect and fulfill that right. The Court adopted a teleological approach noting that “strike action is one of the main activities engaged in and tools used by workers and their organizations to promote their interests and improve conditions of labour thereby ensuring the effective exercise of the freedom of association protected under Convention No. 87.” The Court concluded that protection of the right to strike is encompassed in the protection of the freedom of association provided for in Convention No. 87. Put differently, failure to recognize the right to strike would effectively undermine freedom of association and risk defeating the treaty’s object and purpose. 

International Legal Accountability Under Human Rights Treaties

Structural Considerations of International Human Rights Law

Reciprocity is a bedrock precept of international law that supports the horizontal legal accountability between states since “reciprocal benefits are usually understood to be benefits from the treaty obtained through the compliance of the other party (or parties).” Human rights treaties are distinct since they generate horizontal and vertical accountability because of the particular correlativity of rights and obligations in human rights law. Under an international human rights treaty, obligations are owed by states parties to one another – generating a more traditional form of horizontal accountability between parties who have consented to be bound to one another in respect of the obligations in the treaty. However, the obligations underpinning a human rights treaty are also owed by the state party to rightsholders – its citizens or those under its effective control – thereby generating a form of vertical accountability. 

The foregoing would appear to militate against the need for an identity of the parties given the plurality of duty bearers and rightsholders – no longer a symmetrical set of relationships (even if one had an identity of the States parties). While rightsholders are not formal parties to a human rights treaties, the accountability structure of human rights treaty obligations renders them a type of third party within the treaty structure: not signatories but still more than mere beneficiaries. Viewed from this perspective, seeking an identity of parties under international human rights treaties is virtually impossible.

There are structural similarities which may render obligations under the ICESCR and ICCPR relevant rules for interpreting Convention No. 87 within the meaning of Article 31(3)(c), even absent an identity of the parties. One may argue that the ICESCR and ICCPR are uniquely relevant to Convention No. 87 because all three are human rights treaties (contra Dissenting opinion Judge Xue) and the obligations they enshrine share the same legal structure and generate the same type of legal accountability. In this respect they are adjacent rules or, Koskenniemi put it, adjoining rules.

Substantive Considerations of International Human Rights Law

Related to the structure of human rights obligations is the nature of the obligations at issue when determining a common understanding in international human rights law. As Campbell McLachlan has opined, “Where, however, both treaties have multiple, but not universal membership, the meaning of ‘parties will depend upon the nature of the obligation being interpreted.” (page 399)

A question to interrogate therefore is whether because of the substantive nature of the obligations at issue in human rights law, a distinct approach is warranted in determining common understanding. Many have argued in favour of The Founding Function of Human Dignity in the Universal Declaration of Human Rights in: The Concept of Human Dignity in Human Rights Discourse. At their core, human rights treaties aim to protect values and rights derived from relating to human dignity which are deemed sufficiently important for States to agree to subject their domestic actions or omissions to external scrutiny and review by international bodies. Law is widely understood to fulfill an expressive function, and human rights treaties fulfill an expressive function grounded in the individual interest in integrity and in human dignity. And as the Preamble of the ICESCR attests, they also have a transformative function in challenging power structures and catalyzing change to social norms. Indeed the Preamble of Convention No. 87 recalls that freedom of association is a means of improving conditions of labour and of establishing peace and that together with freedom of expression, is essential to sustained progress.

The foregoing suggests that the particular nature of the rights and corresponding obligations under human rights treaties may introduce a different perspective in the identification of a “common understanding” across international frameworks based on the unique material relevance of the norms at issue: in this case the relevance of the right to strike (ICESCR) and the right to freedom of association (as interpreted under the ICCPR) to the right to freedom of association under Convention No.87. These three treaties are part of the international human rights legal ecosystem: the special nature and affinity of the norms they enshrine undergird their mutual relevance and thus support a common understanding in regard to the right to strike. 

Conclusion

Having (correctly) departed from requiring an identity of the parties in its determination of relevant rules within the meaning of Article 31(3)(c) in this case, the Court relied on the existence of a common understanding of the parties as to the provisions of Convention No.87 in its decision to have recourse to provisions of the ICESCR and ICCPR. 

The analysis above aligns with the approach of the Court and argues additionally in favour of a particular interpretative approach for human rights treaties. Nevertheless, such arguments are not without their limits, and the remit of “common understanding” should not be stretched so far beyond the identity of the parties as to vitiate consent entirely.

Indeed, the strength and sustainability of international legal accountability depends upon clear adherence to the principles of consent and reciprocity. This is the case for international human rights treaties as well – the core of their normative power derives from the fact that they embody voluntarily assumed obligations vis-à-vis other states parties and vis-à-vis their own citizens. There can be no equivocation of consent if a structural integrity of international human rights treaty law is to endure. This would point to the need for a selective and careful use of “common understanding” where there is an absence of an identity of parties to identify “relevant rules applicable between the parties” under Article 31(3)(c).

Thus, the Court was correct in seeking a strong degree of overlap as an essential element in securing a balance that prioritizes the coherence and evolution of human rights norms, but also respects the foundational principle of consent in international law. Nevertheless, the Court left a number of important questions unanswered: how can a rule be distilled from the provision invoked in the external treaty? What constitutes “a high degree of overlap” and what indicators should be deployed to identify it? What are the indicia for establishing “a common understanding”? This note suggests that there are additional factors to be borne in mind when answering these questions in the context of human rights treaties.

A final observation is apposite in view of the geopolitical context in which the Advisory Opinion was issued. At a time when multilateralism and human rights systems (particularly regional) are confronting direct and unprecedented challenges, an interpretative approach that promotes joined up thinking, legal alignment and coherence is particularly welcome.