The Curious Case of the ‘Legal Effect’ of ICJ Advisory Opinions in the Mauritius/Maldives Maritime Boundary Dispute

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On Thursday 28 January, the ITLOS Special Chamber delivered its judgment on the admissibility of the Dispute concerning the delimitation of the maritime boundary between Mauritius and Maldives in the Indian Ocean (Mauritius/Maldives). One of the key issues was the Maldives’ contention that the ongoing dispute between Mauritius and the UK over the Chagos Islands precluded the exercise of the Special Chamber’s jurisdiction. Despite this objection, the Chamber held (with Judge Oxman dissenting) that it did indeed have jurisdiction to proceed to the merits.

There are many features of this case that are worthy of discussion. The oral proceedings were the first in ITLOS’ history in which hybrid procedures were employed in response to the COVID-19 pandemic. The Chamber’s judgment is yet another important nail in the coffin for the UK’s claims to overseas territory and a victory in the fight for decolonisation.

However, this blog post will focus on a more legalistic element, namely what the Special Chamber calls the ‘legal effect’ of Advisory Opinions of the International Court of Justice (ICJ). It will explain and analyse the rather remarkable conclusion of the Chamber that the dispute between the UK and Mauritius had in fact already been determinatively resolved by the ICJ’s earlier Advisory Opinion, and that there was therefore no bar to jurisdiction. It questions the logic of the Chamber’s reasoning in this regard and reflects on the possible implications of this finding.

Background to the case

Most readers will be aware of the situation concerning the Chagos Archipelago and the ongoing disagreement between Mauritius and the UK in this regard. To summarise what is a long and tragic story, the UK made claim to these islands during the colonial era and continues to do so, despite Mauritius’ claims to the contrary.

Mauritius has in recent years made ample use of international legal tools to further its claim to the Chagos Islands. From the UNCLOS Arbitral Tribunal in 2015 (see here), to the ICJ’s 2019 Advisory Opinion (see here), Mauritius has been gradually building up a series of favourable decisions and judgments, bolstering its claim to the Chagos Archipelago.

In the months following the ICJ’s Opinion, Mauritius instigated proceedings before ITLOS in relation to a dispute between itself and the Maldives regarding the delimitation of the maritime boundary between them. As the maritime area between these two states includes the Chagos Archipelago, this may be seen as another effort by Mauritius to assert its territorial claim over these islands.

Preliminary Objections of the Maldives

This is indeed the interpretation of the Maldives, who, in their Preliminary Objections (the first two of which are relevant here), argued that the case was inadmissible because (1) the UK was an indispensable third party under the Monetary Gold rule, and (2) the Tribunal would be required as part of the delimitation decision to determine a dispute over the sovereignty of the Chagos Islands, something that it did not have the authority to do. The Special Chamber considered these two objections together as it found that both were premised upon the contention that there was an ongoing, unresolved dispute over the sovereignty of the Chagos Archipelago. The key question was whether this dispute had been authoritatively resolved, most notably by the ICJ’s 2019 Advisory Opinion.

Arguments of the parties

The two parties disagreed, firstly, whether the 2019 Advisory Opinion had in fact come to a determination as to the disputed sovereignty of the Chagos Archipegalo. The Maldives of course argued that it had not, while Mauritius contended the contrary. While this was indeed an essential element of the case, the focus here is on the legal effect of the Advisory Opinion, assuming that it had indeed determined the sovereignty of the Chagos Archipelago.

The Maldives argued, in line with ICJ case law (e.g. South West Africa cases at p337) that ICJ Advisory Opinions are not binding, and that to accept that they were would be contrary to the fundamental principle of consent to jurisdiction (at [27]). Thus, even ‘if the Court had given advice on the sovereignty dispute…that advice would not have the legal consequence of resolving that dispute’ (at [60]).

Mauritius argued that while such Opinions are not ‘directly’ binding, ‘this does not mean, however, that an Advisory Opinion is devoid of legal effects’ ([3.18]-[3.19]). They submitted that such Opinions are authoritative statements of the law, due to the ICJ’s role as the principal judicial organ of the UN (at [3.21]). Other international courts and tribunals had treated ICJ Advisory Opinions as such (this was disputed by the Maldives). Mauritius concluded, in an interesting – and legally questionable – turn of phrase, that ‘[i]n essence, the Maldives invites the Special Chamber, by way of its Preliminary Objections, to disregard and effectively overrule the ICJ’s authoritative determination that the United Kingdom has no lawful basis to claim sovereignty or sovereign rights in regard to the Chagos Archipelago’ (at [3.28]).

The judgment of the Special Chamber

The Special Chamber found that it was ‘necessary to draw a distinction between the binding character and the authoritative nature of an advisory opinion of the ICJ’ (at [203]). It emphasised that ‘judicial determinations made in advisory opinions carry no less weight and authority than those in judgments because they are made with the same rigour and scrutiny by the “principal judicial organ” of the United Nations’ (at [203]).

While acknowledging ‘that it is beyond doubt that there had been a long-standing sovereignty dispute between Mauritius and the United Kingdom over the Chagos Archipelago’, as confirmed by the 2015 Arbitral Tribunal ([242] in the Special Chamber judgment), the Special Chamber noted that ‘the key question in the present proceedings is whether the legal status of the Chagos Archipelago has been clarified by the advisory opinion of the ICJ.’ If this is the case, according to the Chamber, ‘the continued claim of the United Kingdom to sovereignty over the Chagos Archipelago cannot be considered anything more than “a mere assertion,”’ not a true dispute (at [243]).

As a brief aside, this last point is especially interesting given the established definitions of the term ‘dispute’, most notably the South West Africa cases (at p328), which were cited by the Special Chamber in its judgment (at [243]). This would seem to be a rather twisted application of that jurisprudence given that the focus in the South West Africa cases and subsequent decisions has always been on whether the two states are actually in disagreement (which is clearly the case between the UK and Mauritius), not a qualitative assessment of one side’s position (as the Special Chamber seems to suggest here). As Judge Oxman notes in his Dissenting Opinion (at [28]), these reflect two quite distinct functions behind the application of the term ‘dispute.’

Finally, the Special Chamber found (at [246], emphasis added) that:

‘[t]he determinations made by the ICJ with respect to the issues of the decolonization of Mauritius in the Chagos advisory opinion have legal effect and clear implications for the legal status of the Chagos Archipelago. The United Kingdom’s continued claim to sovereignty over the Chagos Archipelago is contrary to those determinations.’

As such, it declared, there was no ongoing dispute. Rejecting the Maldives’ Objections on this point, the Special Chamber found that it did indeed have jurisdiction.

Authoritative statement vs legal effect

Although undoubtedly interesting, this reasoning of the Special Chamber is somewhat questionable. Whatever the Chamber may say about drawing a distinction between the binding force of a decision and its authoritative statement of law, this obfuscates the real issue. The distinction that matters here is between authoritative statement of law and the legal effect of that statement itself. Saying that the ICJ, as the principal judicial organ of the UN, is qualified to make an authoritative assessment of what the law is, is not the same as saying that this assessment has legal effect, i.e. that it can alter the rights or obligations of international legal subjects. By declaring that the ICJ’s Chagos Opinion has determinatively solved the sovereignty dispute in question, the Special Chamber fails to distinguish between these two separate things.

Despite the absence of a formal system of precedent, the decisions of international courts and tribunals are regularly treated as authoritative – or at least followed – in later cases. In so doing, the court or tribunal in question takes the conclusions of the previous judicial body and applies them to the dispute at hand in order to resolve it. Assuming that the parties to the earlier case are different to those of the later case, the prior decisions do not have a direct legal effect on the latter.

To clarify, then: the Special Chamber in this case did not agree with the ICJ’s statement of the law and, as such, apply it in resolving the dispute between the UK and Mauritius. Indeed, it would not have had the authority or jurisdiction to do so. Instead, it deemed the dispute already solved; it decided that it had been legally concluded by an Advisory Opinion – an Opinion that we all seem to agree has no binding effect. This ruling goes further than even a recognition of a system of precedent. It could be said to recognise a quasi-legislative power on the part of the ICJ, limited only by the infamously blurry distinction between declaring the law and making the law.

Furthermore, even if the ICJ’s Chagos Opinion were binding – e.g. if it had been an inter partes decision between the UK and Mauritius – it is still not apparent that the effect of such a decision would have been to extinguish the sovereignty dispute in its entirety between these two states. Firstly, it is arguable whether the ICJ unambiguously stated that the Chagos Archipelago fell within Mauritius’ territory; the Special Chamber itself only notes that the ICJ’s conclusions ‘have implications for the legal status of’ the Archipelago. Secondly, the ‘legal effect’ of extinguishing such a dispute is not necessarily the same as the ‘legal effect’ of binding the parties to a judgment. While it may indeed be possible for binding legal decisions to have such an effect, this is not immediately self-evident. Thus, even regardless of the binding nature of the decision, the extinguishment of a sovereignty dispute requires a further legal step – one which appears to be missing here.

It is unclear exactly what the implications of this particular aspect of the Tribunal’s decision will be in the longer term. Oxman warns (at [32]) that ‘it risks complicating…the exercise by the ICJ of its discretion with respect to requests for Advisory Opinions.’ From a jurisprudential perspective, much will depend on whether this reasoning is followed by other judicial bodies. Perhaps we will see more decisions granting ‘legal effect’ to Advisory Opinions. Maybe the ICJ itself will address the issue.

But perhaps this is essentially a one-off; a bit of legal trickery or magic that allows the Special Chamber to ignore jurisdictional obstacles rooted in a discredited colonial claim. It would not be the first time that such legal ‘blips’ have been employed by courts where the moral imperative is clear and the legal landscape rather less so (see, for example, my and Craig Eggett’s post on inconsistencies in the ICJ’s Chagos Opinion itself). 

We shall see in due course whether this doctrine of ‘legal effect’ is swept under the metaphorical carpet or whether it heralds a new era of ICJ authority. In the immediate case, however, it means that Mauritius has succeeded in proceeding to a hearing on the merits – and potentially soon adding yet another favourable international legal decision to its growing portfolio

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Marcelo Farewell says

February 5, 2021

It is untenable to criticize ITLOS for giving some legal effect to the AO. Actually, the AO itself makes a request to the General Assembly, and the GA followed, by indicating a date by which the UK had to leave Chagos. Not giving it legal effect would have meant legitimizing the "unlawful act"of the United Kingdom.


February 7, 2021

In physics, there is a metaphor called ”the Schrodinger's cat“, which means you will never know whether the cat is dead or alive until(unless) you watch it. In other words, before you make an observation, the cat is simultaneously in two situations (alive “and” dead, not alive “or” dead). The thing is quite similar to the “legal effect” observed by the ITLOS Chamber: the legal effect of an AO seems not to be determined until one other court (dispute settlement body) applies (identifies) it. In Mauritius/Maldives, it is tricky to observe who declares the sovereignty dispute between UK and Mauritius settled? Whatsoever, it is the Chamber’s interpretation that “Mauritius’ sovereignty over the Chagos Archipelago can be inferred from the ICJ’s determinations”, rather than the ICJ clearly saying the words. If,a big if, the 2019 AO was interpreted by other bodies, the “legal effect” may have been conceived in a different way. Therefore, the “legal effect” of the 2019 AO seems to be constructed by both the ICJ (the one who gives an AO) and ITLOS Chamber (the one who observes the AO) together.

Sarah Thin says

February 8, 2021

Many thanks for your comments, they are much appreciated. Allow me to respond below.

@Marcelo Farewell
First, I am not sure it is entirely correct to say that the AO requests anything in particular of the GA – the purpose of the AO is, in Court’s words, ‘for the General Assembly to receive the Court’s assistance so that it may be guided in the discharge of its functions‘ (at [86]). Indeed, the Court emphasises that ‘it is not for it to determine what steps the General Assembly may wish to take after receiving the Court’s opinion’ (at [179], punctuation omitted). The GA resolution that followed the AO (73/295) did indeed demand the UK’s withdrawal from the Chagos Archipelago and state that the latter was an integral part of the territory of Mauritius. However, as is generally agreed, neither GA Resolutions nor AOs are legally binding. Nor, as I have argued above, is there any logical or legal basis for recognising that they have the ‘legal effect’ of extinguishing the dispute between the UK and Mauritius. This does not mean at all that the UK’s position is legitimate; as I emphasise in my post, the Special Chamber did not decide upon the sovereignty dispute itself – it deemed the dispute already solved. To refuse jurisdiction on this point or on any other would have no bearing on the legitimacy or otherwise of either the UK’s or Mauritius’ claims to the Chagos Archipelago.

@Kaiqiang Zhang
Indeed, I am familiar with Schrodinger’s cat – if I remember correctly it has something to do with waves and particles but I must admit that this is not my forte! I suppose part of the issue here is that (international) law will always, in practice if not in theory, be what (international) courts and tribunals say it is. However, that only heightens the need for clear and well-reasoned legal decisions, and for a clear and predictable theory of law and legal effect underlying those decisions. In that sense it is different from Schrodinger’s cat, if I may stretch this analogy to the breaking point. We should be able to accurately predict, for the most part, whether the cat is alive or dead based on previous examples of cats in boxes and the application of legal logic. The Chamber’s decision here does not, in my opinion, follow such a predictable line of logic but rather departs from it in order to come to a more palatable conclusion than it would otherwise have arrived at.


Marcelo Farewell says

February 9, 2021

In its Advisory Opinion, the ICJ made clear and unequivocal legal determinations that cannot be ignored. First, the Court concluded that, “as a result of the Chagos Archipelago’s unlawful detachment and its incorporation into a new colony, known as the BIOT, the process of decolonization of Mauritius was not lawfully completed when Mauritius acceded to independence in 1968.” (para. 174 of the AO) Second, the Court determined that, “having found that the decolonization of Mauritius was not conducted in a manner consistent with the rights of peoples to self-determination, it follows that the United Kingdom’s continued administration of the Chagos Archipelago constitutes a wrongful act entailing the international responsibility of that State … It is an unlawful act of a continuing character which arose as a result of the separation of the Chagos Archipelago from Mauritius.” (para. 177 of the AO) And third, the Court declared, that, “[a]ccordingly, the United Kingdom is under an obligation to bring an end to its administration of the Chagos Archipelago as rapidly as possible, thereby enabling Mauritius to complete the decolonization of its territory in a manner consistent with the right of peoples to self-determination.” (para. 178 of the AO)

The ICJ added that, “while it is for the General Assembly to pronounce on the modalities required to ensure the completion of the decolonization of Mauritius, all Member States must co-operate with the United Nations to put those modalities into effect.” (para. 180 of the AO) The UNGA acted upon the AO with resolution 73/295, demanding that “the United Kingdom … withdraw its colonial administration from the Chagos Archipelago unconditionally within a period of no more than six months” (op. para. 3 of the resolution). The United Kingdom failed to meet this demand.

As noted by the ITLOS Special Chamber in its Judgment, “it is generally recognized that advisory opinions of the ICJ cannot be considered legally binding”, but “it is equally recognized that an advisory opinion entails an authoritative statement of international law on the questions with which deals.” (para. 202 of the Judgment) The Special Chamber further notes that “judicial determinations made in advisory opinions carry no less weight and authority than those in judgments because they are made with the same rigour and scrutiny by the “principal judicial organ” of the United Nations with competence in matters of international law.” (para. 203 of the judgment)

This is supported by rich literature on the legal effect of ICJ advisory opinions. For example, Rosenne observed that “[t]he fact that the advisory opinion has no binding force … nevertheless does not confer upon the statement of law contained in an advisory opinion characteristics any different from those of the statement of law contained in a judgment.” He further stated that “the substantive problems of the post-adjudicative phase of an advisory case are not dissimilar to those of the post-adjudicative phase of a contentious case. In both instances the Court has declared the law.” Abi-Saab, quoted by Kolb, also observed: “An advisory opinion is not just advice or consultation […] There is no fundamental difference between the intrinsic value of the content of the Court’s opinion and that of a judgment given by that same Court, in the sense that both are authoritative judicial pronouncements deciding questions [of law] that have been submitted to the Court.”

In my view, the ITLOS Special Chamber was absolutely right not to disregard, but take into account, the clear and unequivocal legal determinations made in the ICJ Advisory Opinion when assessing the legal status of the Chagos Archipelago in its Judgment.

Regards, M.

Sarah Thin says

February 11, 2021

Dear Marcelo,

Thanks once again for your contribution. I do not dispute (for these purposes at least) the authorititative nature of the ICJ's statement of the law. The scholarship that you quote supports this contention - in simple terms, that when the ICJ gives its view on what the law is, the ICJ is right, regardless of whether it does so in the form of a binding decision or an AO. The issue is not with the accuracy of the determination of the law, but with its legal effect, i.e. its ability to alter the rights and obligations of legal subjects. *This* is the issue, and I do not think the 'legal effect' given to the ICJ's AO by the Special Chamber in the Mauritius/Maldives decision is implied by any previous decisions on ICTs, nor by the scholarship that you raise in your comment. I hope this is clear.

Marcelo Farewell says

February 11, 2021

Dear Sarah,

Thank you for the chance of this debate.

Your position is clear enough, although I do not share it. I find your reading of the literature and your view of the potential legal effect of the AO quite narrow. I do not think it takes into account the unique circumstances of this case, which are well articulated in the comprehensive judgment of the Special Chamber. In particular, 1) Mauritius has enjoyed important maritime rights in respect of Chagos pursuant to the Lancaster House undertakings of the UK from 1965 and Chagos has thus been subject to a special regime (paras. 139 and 246 of the judgment); and 2) the decolonization and sovereignty of Mauritius, including Chagos, are inseparably related, as also implied by the ICJ in para. 88 of the AO (para. 189 of the judgment). Consequently, the determinations made by the ICJ regarding the decolonization of Mauritius have clear and unmistakable implications for the legal status of Chagos (para. 246).


Richard P Dunne says

February 12, 2021

Thin dwells on what she calls the "legal effect" of the ICJ's "authoritative statement of law", interpreting her use of the phrase "legal effect" as meaning "that it can alter the rights or obligations of international legal subjects". In so doing she confuses 'legal effect' with 'binding legal effect'. Whilst it is clear that the AO does not have a direct binding legal effect that does not mean that it has no "legal effect".
The ICJ AO was declaratory of the customary international law (CIL) of self-determination as it existed in 1965. The 'legal effect' of such a declaration is that Britain's failure to grant this right to its colony of Mauritius in 1965 meant that the detachment of the Chagos in 1965 was unlawful. If unlawful, it follows that Chagos remained a part of the British colony of Mauritius until 1968 when the colony became independent. BIOT thus never had a legal existence as the ICJ concluded. So in this sense the natural and logical consequence was to determine sovereignty although the AO could not bind Thin's "international legal subjects".
It would have been surprising if ITLOS did not follow the ICJ AO's declaration of CIL and the ‘legal effect’ of this. There is nothing curious or inconsistent in its arguments. ITLOS’s decision has ‘binding legal effect’ on Mauritius and Maldives but it goes further than that since it is declaratory of the ‘coastal State’ in the Chagos for the purposes of UNCLOS.
The UK continues to adopt the position that it is still not bound by either the ICJ AO or the ITLOS judgment and remains sovereign over Chagos. It is living in a world of make believe which is destabilising to world order. Will it continue to try to arrest and prosecute illegal fishing in the Chagos 200nm fishery zone? Will it intervene if a Mauritian vessel sails into Mauritius’ territorial sea in the Chagos?
Britain pretends to uphold international law and humanitarian law but in reality behaves as a petulant bully.

John R Morss says

February 16, 2021

Excellent piece imho. A brief comment on Richard if I may: Britain's 'failure to grant' the retrospectively identified CIL-(and other sources)-based right to self-determination to Mauritius in 1965 can hardly be said to have caused/had the effect that the Chagos detachment was unlawful. Surely it was the other way around... the detachment and the formation of BIOT and especially the lease of Diego Garcia to USA, constituted so to say a clog on the equity of redemption (haven't used that phrase for a while) ie in hindering the carrying forward as and when, of the decolonising/self-determination-facilitation process?...

Richard P Dunne says

February 16, 2021


It seems fairly clear to me.

The ICJ first established that the law of self-determination constituted the applicable CIL in 1965, and went on to conclude by reference to Resolution 1514(XV) para 6 that at that time: “any detachment by the administering Power of part of a non-self-governing territory, unless based on the freely expressed and genuine will of the people of the territory concerned, is contrary to the right to self-determination” [para 160].

It found that Chagos was an integral part of the non-self-governing territory of Mauritius in 1965 prior to its detachment [170] and that the Lancaster House Agreement with the Council of Ministers was not based on the free and genuine expression of the will of the people concerned [172]. In consequence it determined that the detachment in 1965 was unlawful [174] ab initio.

Sarah Thin says

February 18, 2021

Dear all,

Thank you very much for your comments! I appreciate the chance for a discussion.

A brief response to Richard:

Regardless of how legal effect and binding legal effect are defined, there is no legal basis upon which the ICJ AO would have such an effect. It is one thing to say that the ICJ can give an authoritative statement of the law (although even this I think is going quite far and strays into announcing a system of precedent); it is quite another to say that this statement would have the effect of extinguishing a legal dispute over territory. A step is missed. The ITLOS Chamber did not (could not have) followed the ICJ’s statement of the law in that sense that it would apply the law as stated by the ICJ in its AO in solving the dispute: it deemed it already solved. There is nothing in previous case law to indicate that ICJ AOs could have this effect. There is nothing in the purpose behind ICJ AOs (to provide legal advice to international institutions, particularly the UNGA) to indicate that ICJ AOs could have this effect. As such, I maintain that this is indeed ‘surprising’. It would not have been surprising at all if the ITLOS Chamber had decided otherwise – as indeed the arbitral tribunal did a few years prior.

‘The UK continues to adopt the position that it is still not bound by either the ICJ AO or the ITLOS judgment and remains sovereign over Chagos.’ – To be honest, I think this is a perfectly correct position to take. It is not bound by either.

The immorality of the UK’s position on the Chagos Islands does not (or at least should not) have any impact on the legal questions relating to the competence of international courts and tribunals. I am no fan of UK colonialism, but on this legal question that is neither here nor there. Indeed, the fact that the UK maintains this position is arguably evidence of an ongoing legal dispute. Presumably the UK would dispute Mauritius’ interpretation of the legal effect of the ICJ AO – there, arguably, is another legal dispute that would prevent the ITLOS Chamber from exercising jurisdiction in the present Mauritius/Maldives case as per the Monetary Gold rule. I find it difficult to escape the conclusion that the Chamber is really unprecedented (in the common sense of the term).

Many thanks to John for your kind words and contributions!


Marcelo Farewell says

February 26, 2021

Dear all,

I thought it might be of interest to note what Judge Rosalyn Higgins has to say about legal consequences or effects of advisory opinions. In her Separate Opinion in the Wall case, she stated:

"Although in the present case it is the Court, rather than a United Nations organ acting under Articles 24 and 25 [of the Charter], that has found the illegality; and although it is found in the context of an advisory opinion rather than in a contentious case, the Court's position as the principal judicial organ of the United Nations suggests that the legal consequence for a finding that an act or situation is illegal is the same". (para 38).

Best to all,