August 19, 2025
By Mónica Ávila Currás
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To celebrate the 15th anniversary of the Strasbourg Observers Blog, we organised an in-person symposium with scholars, practitioners, and members of the ECtHR on 8–9 May 2025 in Ghent. Connecting in person with so many regular contributors was a wonderful experience and led to engaging dialogue with current and former judges of the Court. To allow our online community to participate in the thought-provoking discussions that took place, we will be publishing selected position papers on the blog over the next two summer months. We wish all of our readers a well-deserved break with a little peek on Strasbourg Observers here and there.
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European states’ practice of intervening in cases before the European Court of Human Rights (ECtHR) with which they do not have an a priori relation has increased over time. A frequently invoked justification for these ‘non-related’ interventions by states is that the Court’s case law has the potential to have an impact beyond the parties to the case. If the interpretation of the ECHR is going to (potentially) have a de facto erga omnes effect, the rest of the Contracting parties may have a vested interest in the case. In this blog post, I will analyse how the de facto, but not de lege, erga omnes effect of the ECtHR judgments has created a systemic tension in the ECtHR system. I propose that, while this tension has been remedied by allowing all Contracting States to act as third-party interveners in any case without many restrictions, in the context of migration at least, this has opened the door to the use of the third-party intervention mechanism beyond what is required to alleviate it.
Article 36(2) of the European Convention on Human Rights (ECHR) recognises the possibility for states that are neither the respondent state nor the state of nationality of the applicant to intervene in cases before the ECtHR if the President of the Court allows them. Although the first ‘non-related’ intervention was in 1979 in the Winterwerp v. the Netherlands (Bürli, p. 134), there was still no legal backing for it at that time. This possibility was not introduced into the Rules of the Court until 1982, and into the ECHR until 1998.
While third-party interventions (TPIs) by states have become more common since 2002, they have remained relatively rare. Although scholars have pointed out that a relatively high number of TPIs are made in migration cases (Bürli, p. 143; Glas, p. 547), my analysis of the caselaw available on HUDOC revealed that since 2008, there has been at least one TPI in only 30 migration cases. For the purposes of this analysis, I consider migration cases those applications brought by migrants, asylum seekers, or refugees in matters related to their migratory status, including situations of (collective) expulsions, extraditions, international protection rejection, immigration detention, family reunification, etc.
My analysis shows that in the context of migration, only one or two ‘non-related’ states intervene per case. The five cases which have attracted the most interveners – above 6 state interveners – were all decided since 2020, with the most recent examples being C.O.C.G. and Others v. Lithuania, H.M.M. and Others v. Latvia, and R.A. and Others v. Poland, where respectively 8, 9 and 11 non-related Contracting Parties submitted a third-party intervention. This suggests a growing interest in intervening in certain types of cases.
As prescribed by Rule 44(5) of the Court, the President of the Chamber sets the rules for the intervening states. After third-party interveners express their wish to intervene, they are sent a letter by the Court. This letter explains that they should not discuss the facts or merits of the case in their submissions to the Court (Leach, para 2119). This requirement is in line with what Paul Harvey, a lawyer in the Registry, considered as ‘effective interventions’: those that provide extra information out of a desire to assist the court. He mentions as examples of ‘effective interventions’ those that provide scientific information to prove an indirect discrimination and those focused on international and comparative law. He stresses, discussing all types of third-party interveners, that the most effective interventions are those where the interveners do not seek to advance their own interests. However, in some of the letters I have been able to access through the registry, the Court requests the states that want to act as third party interveners to submit ‘aspects which concern the Government’s particular interests in this matter’ which could be considered ‘in their own interest’. Thus, while other third-party interveners are not encouraged to do so, states are allowed, and even encouraged to, share their own particular interest on the matter.
And intervening states probably do as they have been requested. My analysis shows that at the beginning of their written submissions or in their speeches during public hearings, third-party state agents either illustrate how their state is connected with the case by stating that it has an impact on their domestic legal framework, or that they are in a similar situation as the respondent; or present themselves as a true amicus curiae that will assist in answering the questions raised or discuss the general principles. This self-interest of the state agents behind their TPIs has been confirmed by many authors: Harvey and Pastor Vilanova have argued that states mainly intervene when they consider that their legal system will be affected by the outcome (Pastor Vilanova, 2019, p. 381; Harvey), or when they try to persuade the Court to interpret the ECHR in a way favourable to them (Dzehtsiarou, p. 383).
Most academic work on the subject of TPIs does not consider it problematic that states may act out of self-interest when entering ‘non-related’ interventions. Glas (p. 542, 557, 560), Bürli (p. 134) and Pastor Vilanova (2019, p. 381) view TPIs from a dialogical perspective. They encourage their use as they see them as a way to facilitate direct engagement between states and the Court. The most repeated justification in the literature is the de facto erga omnes effect and the res interpretata effect of the ECtHR judgments. Proponents of this view stress that the Contracting states have a legitimate interest in the Court’s interpretation of the Convention. While ECtHR judgments are formally binding only on the parties to a case, those establishing new legal principles or standards should have a persuasive authority for all states (Glas, p. 557). Such landmark cases may thus produce what has been described as a de facto erga omnes or res interpretata effect, influencing states beyond the immediate dispute.
Conversely, another part of the literature approaches non-related TPIs more critically. In the context of migration, Baumgärtel highlights the potential for states to engage in collective third-party interventions as a means of mitigating the influence of European courtsSubsequently, Dzehtsiarou explicitly refrains from advocating for such interventions, acknowledging that while they might provide the Court with a broader range of perspectives, they present significant challenges. He argues that an increase in the number of interventions would require additional ECtHR resources process them (p. 383) and suggests that a high volume of state interventions could affect the Court’s decisions (p. 400).
Both perspectives offer valid insights. A ‘dialogical perspective’ offers a solution to the instability created by a de facto, but not de lege, erga omnes effect of some ECtHR judgments – as will be discussed next, and a more ‘critical perspective’ stresses the risks associated with this solution, which will be addressed at the end of the blogpost.
The erga omnes effect in the context of international courts and adjudication is used to describe the effects of judicial decisions that affect third parties, and is usually defined in contrast with the inter partes effect (Tams and Belle, para. 1). It ‘connotes the idea that the interpretative authority of international judgments reaches beyond the parties to the case.’ (Arnardóttir, p. 822)
Due to the explicit reference to the inter partes effect of the Court’s judgments under Article 46 ECHR, it would be hard to argue that ECHR judgments have a de lege erga omnes effect (Tams and Belle, para. 6). In fact, during the Interlaken Reform process that took place from 2010 to 2019, the possibility of recognizing the legally binding erga omnes effect of some judgments – notably those produced by the Grand Chamber – vis-à-vis all Contracting parties was discussed (Arnardóttir, p. 823-824), but in the Steering Committee for Human Rights (CDDH)’s final report, that idea was strongly rejected (paras. 37, 64). Instead, they embraced the res interpretata principle (paras. 40-42).
Generally, the principle of res interpretata refers to ‘the binding force of an interpretive finding’ (Tams and Belle, para 2). In the context of the ECtHR, however, it refers to how, even though the ECHR does not contain a doctrine of binding precedent, the reality is that ‘once the ECtHR has pronounced on an issue, it is to be expected that the Convention will be interpreted and applied in the same manner if the Court is confronted with the same issue again in a different state’ (Arnardóttir, p. 823). This principle emerges from the application of Articles 19 and 32 of the ECHR, taken together with Article 1,and is implied by the Court in its case law by saying that its judgments are meant not only to resolve individual cases but also to ‘elucidate, safeguard and develop the rules instituted by the Convention’ (Rantsev v. Cyrpus and Russia, para. 197). The Contracting Parties defend this principle as well. The Action Plan that resulted from the Interlaken Process, which contains the joint statement of the Contracting parties towards the future, mentions this idea too (Tams and Belle, para. 26).
Despite the contradictory positions in the CDDH’s final report on the erga omnes versus res interpretata effects, the res interpretata effect is in practice fundamentally linked with a de facto erga omnes effect. Arnardóttir describes how the res interpretata effect ‘introduces a limited de facto erga omnes partes effect for the judgments of the Court’ (Arnardóttir, p. 820), meaning in practice that international courts have the ability to settle interpretations that will be binding not only to the parties of the case, but also having a de facto erga omnes effect across all the Contracting States (Arnardóttir, p. 823)
This feature is not specific to the ECtHR. Courts specialised in one treaty have a massive influence on its substance (Tams and Belle, paras. 24-26). In that sense, the ECtHR has the authority to decide upon the interpretation of the ECHR, as it is considered the ‘all-controlling gatekeeper of the Convention’s meaning’ (Tams and Belle, para. 25).
This special nature of the ECtHR case law as an authoritative interpreter of the ECHR with effects vis-à-vis all Contracting States has been expressed by the Court itself (Arnardóttir, p. 822; Pastor Vilanova, 2018, p. 120). And despite the CDDH final declaration refusing the erga omnes effect of the ECtHR’s judgments, the Contracting Parties seem to have accepted it de facto when they do modify their domestic legislation after ECtHR’s decisions on cases they were not part of (Tams and Belle, para. 25).
How is this apparent contradiction between, on the one hand, the de facto erga omnes effect produced by the res interpretata effect of at least some of the ECtHR’s judgments and, on the other hand, the reluctance of states to establish a de lege erga omnes effect, resolved? This blog asserts that third-party interventions by states function as a systemic response to a legal-institutional gap raised by this contradiction in the ECtHR system. I argue that as a result, the initial possibility to intervene provided to Contracting States under 36(2) ECHR is being transformed into an almost unrestricted right to intervene due to the reported practice of the ECtHR to always allow interventions from states (Glas, p. 542).
Since the beginning of this blogpost, it has been stressed how most of the authors discuss that only some judgments have res interpretata or de facto erga omnes effect (Glas, p. 557; Pastor Vilanova, 2019, p. 382).
Acknowledging the common use of this disclaimer, Pastor Vilanova poses the following question: ‘Do all judgments from the Court have erga omnes effect? If not, how to identify which ones have it?’ (Pastor Vilanova, 2018, p. 120). He suggests that all Grand Chamber judgments should have a de facto erga omnes effect, along with the Chamber judgments that the Court itself has considered a landmark judgment (Pastor Vilanova, 2018, paras 120- 121). This seems to be in line with the spirit of the Contracting Parties to the Interlaken Process, as Arnardóttir mentioned how it was precisely a possible de lege erga omnes effect of the Grand Chamber judgments that was proposed (Arnardóttir, p. 823), but not of all.
In the next section of this blogpost, I will analyse ‘a posteriori’ – that is, after the third party intervention was submitted, and the judgment or decision rendered – the justifications for TPIs based on the de facto erga omnes effect in the practice. Drawing on data collected for my dissertation and focusing on migration cases, I will first examine whether ‘non-related’ states have intervened in Grand Chamber cases or in cases that would later be considered landmark. Second, I will assess -only through an example – if the intervening states’ legal framework are likely to be affected by the de facto erga omnes partes of the interpretation given in those judgments
There is no consensus on what a landmark judgment is in the ECtHR context, but rooting on Pastor Vilanova’s proposal – to consider landmark those self-assessed as such by the Court – I will use as a reference HUDOC’s ‘key cases’ and the Guide on the case-law of the European Convention on Human Rights on Immigration and the Guides on Article 3, 8, 13 and 14 – the ones claimed in these cases – of the European Convention of Human Rights.
In the frame of my dissertation, I identified all the judgments and decisions that have been produced so far, and are available in HUDOC, related to a migration case where a European state intervened before the ECtHR. Until November 2024, ‘non-related’ states had intervened in 27 migration cases1 with a total of 64 interventions:2 40 were produced before the Grand Chamber3, and 24 before a Chamber. Only five of the twenty-four submissions sent before a Chamber judgment concerned judgments that are considered key in the HUDOC database or included in the ECtHR’s Guides mentioned above.
| Grand chamber | Chamber | ||
| Key cases | Not key cases | ||
| Number of Interventions | 40(1) | 5 | 19(3) |
*The numbers in parentheses represent the submissions by states in support of the applicants, NOT the respondent State.
The results show that when ‘non-related’ states intervened in migration cases, in 30% of the cases the submissions were issued for neither Chamber judgments considered key nor Grand Chamber judgments. It is impossible for the states to know upfront which cases will become landmark, they take a risk. It is still interesting, though, that in nearly a third of the cases, states intervened in proceedings that were ultimately inadmissible, struck out, or lacked legal significance.
If allowing states as third-party interveners resolves the tension between the de facto erga omnes effect and Article 46’s inter partes effect, it is justified only if they are impacted by the case’s provisions’ interpretation. There is at least one exception to this.
In N.D. and N.T v. Spain, a landmark case on the collective expulsions of migrants at external land borders, France, Belgium, and Italy intervened before the Grand Chamber to defend a concrete interpretation of Article 4 Protocol 4 that would include a check on what has been called ‘the culpable conduct’ of the applicant (Ávila Currás, p. 293). Out of these countries, only France, in French Guiana, has land borders with countries outside the European Union or the Schengen area. Neither Belgium nor Italy’s domestic legal frameworks would be impacted by the interpretation given by the Court of the Article 4 Protocol 4 in land border cases. Only if the arguments proposed by them are expected to have an impact beyond similar situations – if they advance the analysis of the culpable conduct to potentially impact another type of cases, for example –, or if they consider they can participate in all cases concerning external borders of the Schengen area, would it make sense for them to intervene to shape the interpretations and definitions crystallized in that Grand Chamber judgment. However, these would go beyond the justifiable with the de facto erga omnes effect.
It is important to mention that my focus here is on the dynamics at play, and how the states, as actors that intervene in the ECtHR system and are constrained by its rules and their own interests, can end up using third-party interventions in ways that go beyond their original function as a means to address specific systemic tensions. The data shows that European states are not necessarily and only intervening in what will become the Court’s authoritative interpretation in situations that would impact their domestic legal frameworks. Although unintended – since, as mentioned, states cannot predict which cases will ultimately be regarded as landmark – they are intervening in cases that will not end up having a de facto erga omnes effect. Moreover states occasionally intervene in matters that do not have an impact in their domestic legal framework, therefore lacking that justification to intervene. This is not to suggest that States always make fully conscious strategic choices; rather, these patterns emerge from the way the system is structured.
The ECtHR’s system gives states ample leeway to intervene without much content restrictions, Yet, by doing so without a clear legal justification, they risk shifting toward a strategic – or even political – use of a procedural tool that was not meant to be used beyond resolving the tension .
Firstly, while aligning with the ECtHR’s conditions set in the letters, it is worth mentioning that some states justify their interest in the matter using certain anti-immigration rhetoric, citing how ‘population movements and migratory flows affect many European countries’, or the acuteness of the ‘national security’ problem to the Contracting States. These justifications are shaped by ideology: they assume that all European states should have a say on migration because migrants are seen as threats that need to be controlled across borders. While this idea has become a common place in political debates, it remains questionable – to say the least- from a human rights perspective. States’ rhetoric around risks posed to majorities is difficult to sustain within a system fundamentally designed to protect minorities.
Secondly, as suggested by Dzehtsiarou, a high number of interventions in a case may add pressure to the Court. Not knowing if the case will end up having a de facto erga omnes effect, the states intervene, moved by the importance of the matter for them – as they also state in their submissions. Having a number of third-party interventions from states in a case gives a sign to the Court of its importance for them, in a similar way to the letter recently sent by nine governments to the Court, which was undisputedly considered a form of political pressure on the Court. Writing a public letter signed by nine states, or submitting nine third-party interventions in a migration case don’t differ as much – merely on the publicity they have. As the United Kingdom’s speech in the hearing of M.N. and others v. Belgium (1:09):
“[I]t is to be noted that 11 states have chosen to intervene and to make submissions (…). That number is itself an indication of the degree of concern among Contracting States about (…). There is both a very wide degree of common reasoning and unanimity in their conclusions.”
This blog argues that third-party interventions by states function as a systemic response to a structural gap in the ECtHR system. My aim is not to argue for abolishing third party interventions by states, as I acknowledge the use they have in balancing this tension, but to observe that the ECtHR current system and structures does not easily accommodate the potential unintended consequences of such interventions. Using Wessel’s ‘productive instability’ concept (pp. 233-252), I suggest that this instability could, in fact, be used productively for European states, above all in an increasingly turbulent political context towards migration, by enabling them to exert a form of collective pressure and to contribute to normalising nativist rhetoric inside the Court. We can only wonder: will the ECtHR be able to withstand the dominant anti-immigration rhetoric and the political pressure from a number of states closing ranks against what is today the most targeted minority?
Strasbourg Observers is an academic blog that discusses recent developments at the European Court of Human Rights. The editorial team of Strasbourg Observers is based at the Human Rights Centre of Ghent University and the Centre for Government and Law of Hasselt University.
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