The Chagos Advisory Opinion and the Decolonization of Mauritius

Stephen Allen
April 15, 2019

In Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965,[1] the International Court of Justice (ICJ) delivered its Advisory Opinion on the two questions posed in the UN General Assembly's request.[2] First, had Mauritius' decolonization been completed when it gained independence in 1968, after  the excision of the Chagos Archipelago? Secondly, what were the legal consequences flowing from the United Kingdom's continued administration of the Archipelago? To answer the request, the Court would need to decide how, and when, the right to self-determination crystallized as a matter of customary international law (CIL). The timing here was crucial. If this CIL norm emerged before 1965, the United Kingdom was under an obligation to maintain Mauritius' territorial integrity, pending the exercise of the right by the Mauritian people. However, if the right to self-determination acquired CIL status after Mauritian independence, then the United Kingdom had the authority to partition the colony in 1965 as a matter of international law. 

It was feared that the Court might shy away from giving an Opinion in this case as, arguably, it concerned a bilateral sovereignty dispute which the United Kingdom had not agreed to have resolved by judicial decision. However, as it turned out, the Court delivered surprisingly robust responses to the questions posed. Advisory Opinions are not legally binding on state parties but they do have considerable weight as they confirm the international legal position in a given situation. This Insight  will analyze the Opinion's findings before considering the ancillary issues that will now need to be addressed by the Assembly.


In 1964, a U.K./U.S. survey of the Indian Ocean revealed that Diego Garcia, in the remote Chagos Archipelago, would make an ideal site for a U.S. military base. The Archipelago was a distant dependency of the Non-Self-Governing Territory of Mauritius and home to the Chagossians.[3] In 1965, the United Kingdom entered into the "Lancaster House Agreement" with Mauritian ministers to detach the Chagos Islands from Mauritius. It applied considerable pressure to obtain consent and the transaction was tied up with the decision to grant Mauritius its independence. The British Indian Ocean Territory (BIOT)was created in 1965 and the Chagos Islands were separated from Mauritius accordingly. In 1966, the United Kingdom and United States concluded a treaty regarding the use of Diego Garcia. Pursuant to the planned construction of a U.S. base on this island, the Chagossians were forcibly removed from the entire Archipelago. Most were transported to Mauritius where they have experienced chronic impoverishment.[4]   

Mauritius has demanded the return of the Chagos Islands for decades. In 2010, it instituted proceedings under the UN Convention on the Law of the Sea (LOSC) in response to the United Kingdom's declaration of a huge Marine Protected Area (MPA) around the Archipelago.[5] Mauritius tried to shoe-horn its sovereignty claim into this case; however, in its Chagos Award , the Tribunal ruled that it lacked the jurisdiction to adjudicate this specific claim.[6] Nonetheless, it decided that the MPA's creation had violated the Convention's provisions.[7] Subsequent bilateral discussions came to nothing and so Mauritius took the matter to the UN General Assembly.[8]       

Jurisdiction and Discretion

The Court found, unanimously, that it had the jurisdiction to give an Opinion but the issue of whether it should decline to answer the request was less straightforward. The danger was that, by responding, the Court might circumvent the principle that bilateral disputes can only be adjudicated on the basis of consent.[9] In the end, the ICJ decided that this risk was overcome by its finding that the dispute was, fundamentally, about decolonization—a process in which the Assembly has performed a leading role. Consequently, the Court held, (12/2), that answering the request would not compromise the integrity of its judicial function. 

There can be no denying that Mauritius and the United Kingdom have been engaged in a long-standing sovereignty dispute concerning the Archipelago. However, the bilateral nature of this contest was played down by the way in which the Assembly's questions were formulated. In key respects, the approach adopted by Mauritius in this case was reminiscent of the strategy pursued by the Philippines in the South China Sea Case.[10] In that case, the Tribunal did not have the jurisdiction to engage in a process of maritime delimitation under the LOSC. However, by determining the character of the maritime features (e.g. islands and rocks) in the affected area, it indirectly ruled on China's sovereignty claim. Likewise, in the Chagos Case, by accepting that the bilateral dispute was inseparable from the issue of Mauritius' decolonization, the Court effectively resolved the question of sovereignty. The reasoning being that once the process of Mauritius' decolonization has been completed by the return of the Chagos Islands, no sovereignty dispute remains outstanding. 

Self-determination and Customary International Law 

The Assembly's first question indicated that the relevant international law was reflected in a number of its own resolutions. In particular,1514(1960)(XV) declared that: all peoples have the right to self-determination (paragraph 2); the colonial powers cannot place fetters on independence (paragraph 5); and any disruption of the territorial integrity of a country is incompatible with the Charter (paragraph 6).[11] However, during the advisory proceedings, the United Kingdom and the United States contended that the right to self-determination emerged as a CIL norm with the adoption of resolution 2625(XXV)(1970)—i.e. after Mauritius had become independent.[12]

But did these resolutions reflect CIL at the material time? Custom evolves through the coincidence of state practice and opinio juris (states behave in a certain manner because they believe that they are obligated so to do). Assembly resolutions are not creative of law per se, but their content may contribute to the formation of CIL. The requisite standard, at the time, was that the practice had to be extensive and virtually uniform.[13] It is widely acknowledged that resolutions may constitute evidence of opinio juris , but whether they amount to manifestations of state practice as well has been more controversial.[14] Nevertheless, it is now increasingly accepted that the activity of states in connection with the adoption of such resolutions (e.g. acts of voting and accompanying statements) may qualify as practice.[15] So, did 1514 generate custom or did this only happen with the passing of 2625? It is notable that the latter was adopted by consensus while 1514 was passed by 89 votes with none against and 9 abstentions—largely the colonial powers. 

The ICJ ruled that the adoption of 1514 marked the crystallization of the right to self-determination in CIL.[16] The Court also took the view that the subject of this entitlement—the "people"—must be defined by reference to the colonial unit as a whole. Moreover, in keeping with paragraph 6 (above), it held that maintaining the territorial integrity of a Non-Self-Governing Territory was a corollary of the right to self-determination. Invoking the formulation it had articulated in the Western Sahara Case ,[17] the Court observed that after 1960 the partitioning of such a Territory would violate the right to self-determination, "unless it is based on the freely expressed and genuine will of the people of the territory concerned."[18]

Applying this test to the circumstances surrounding the negotiation of the 1965 Lancaster House Agreement, the ICJ found that, under the 1964 Mauritian constitution, the Mauritian leaders did not have the authority to agree to the Archipelago's detachment. Further, it observed that a high level of scrutiny was required for the purpose of assessing whether consent had actually been given.[19] In effect, the Court held that consent could only have been obtained through a referendum held before the decision was made. It, therefore, reached the conclusion that Mauritius' decolonization had not been lawfully completed in 1968.[20]

State Responsibility 

The Court then turned to the Assembly's second question—the legal consequences flowing from its answer to the first one. It decided that the United Kingdom's continuing administration of Chagos Islands constitutes an international wrongful act for which it bears responsibility.[21] In addition, the Court reaffirmed its view that self-determination manifests anerga omnes character (i.e. all states have a legal interest in safeguarding this right).[22] To this end, the ICJ invoked UN General Assembly resolution 2625, which declares, amongst other things, that all states are under a duty to cooperate with the UN regarding the realization of the principle of self-determination. However, it stopped short of holding that self-determination is a peremptory, or higher, norm of international law (jus cogens ), a finding that would have triggered a duty of non-recognition on all states, as far as the illegal situation in the Chagos Archipelago is concerned.[23] Judges Trindade and Robinson, in their Separate Opinions, argued that the ICJ should have endorsed self-determination's peremptory nature. However, even without such a ruling, states are under a duty not to knowingly assist the United Kingdom in the perpetuation of its internationally wrongful act. If they do so, they run the risk of being found complicit in this breach of international law.[24]

Mauritius was careful not to set out the full ramifications of the Assembly's second question, beyond demanding the return of the Archipelago. In part, this may have been in order to avoid the dispute being characterized as a bilateral one. If the case was about decolonization, then Mauritius could hardly be seen to be claiming hefty reparations. The main Opinion did not consider the question of reparations but it must surely now be in the minds of Mauritian government. In this respect, it is arguable that the United Kingdom's internationally wrongful act manifests a continuing character, and its responsibility was triggered in 1965 when the BIOT was founded.[25]

Next Steps

Beyond calling on the United Kingdom to withdraw from the Archipelago as rapidly as possible, the ICJ was careful to leave the modalities for the completion of Mauritius' decolonization to the Assembly.[26] It will be interesting to see how the Assembly reacts. It may call upon states to withdraw their recognition of the United Kingdom's administration of the Archipelago. Such collective obligations are usually declared by the Security Council, but the Assembly might adopt a novel approach to this situation given self-determination's erga omnes quality and the fact that the United Kingdom holds a veto power in the Council.[27] What is the Opinion's significance for the Chagossians? The Court was content to leave the issue of resettling the Archipelago to the Assembly and, thus ultimately, to Mauritius. Finally, what of the future of the U.S. base on Diego Garcia? The Opinion was silent on this point but Mauritius has undertaken to allow the current arrangement to continue. The U.S./U.K. agreement is due to end in 2036 and so the fate of the base looks uncertain in the longer term.

About the Author: Stephen Allen is a Senior Lecturer in Law at Queen Mary, University of London, and a barrister at 5 Essex Court Chambers, London.

[1] Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965, Advisory Opinion (Feb. 25, 2019),, [hereinafter Opinion]. 

[2] G.A. Res. 71/292 (June 22, 2017).

[3] Mauritius was listed as a Non-Self-Governing Territory in G.A. Res. 66(I), December 14, 1946. See UN Charter art. 73.

[4] See Stephen Allen, The Chagos Islanders and International Law (2014).

[5] United Nations Convention on the Law of the Sea, Dec. 10, 1982, 1833 U.N.T.S. 39.

[6] Chagos Marine Protected Area (Mauritius v. U.K.), Award (Annex VII LOSC Tribunal, Perm. Ct. Arb. 2015). However, Judges Katega and Wolfrum, reached the opposite conclusion.

[7] In particular, Articles 2(3), 56(2), and 194(4).

[8] Although the Tribunal decided that the MPA's creation breached international law, for the time being, it remains in force as a matter of U.K. law.

[9] See Western Sahara, Advisory Opinion, 1975 I.C.J. Rep. 12, 32–33 (Oct. 16), [hereinafter Western Sahara].

[10] South China Sea Case (Philippines v. China), Merits Award, (Annex VII LOSC Tribunal, Perm. Ct. Arb. 2016).

[11] G.A. Res. 1514(XV) (Dec. 14, 1960). Also see G.A. Res. 2066(XX) (Dec. 16, 1965) on Mauritius.

[12] G.A. Res. 2625(XXV) (Oct., 24, 1970). See, e.g., Transcript, Public Sitting on the Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965, at 45 (Sept. 3, 2018),

[13] North Sea Continental Shelf Cases (Germany v. Denmark and Netherlands) 1969 I.C.J. Rep. 3, 43 (Feb. 20).

[14] See International Law Association [ILA], Statement of Principles Applicable to the Formation of General Customary International Law 41 (2000),

[15] See id. at 19, for the ILA's view regarding the practice of international organizations as "state practice."

[16] Opinion, supra note 1, at 150–153.

[17] Western Sahara, supra note 9, at 59.

[18] Opinion, supra note 1, at 160.

[19] Id.  at 172.

[20] Id.  at 174.

[21] Id.  at 177. 

[22] Id.  at 180. See East Timor (Portugal v. Australia) Judgment, 1995 I.C.J. Rep. 102, 29 (June 30).

[23] See  International Law Commission's Articles on Responsibility of States for Internationally Wrongful Acts, arts. 40, 41(2) (2001).

[24] See id. art. 16.

[25] See id. art. 14.

[26] Opinion, supra note 1, at 178–179.

[27] The U.K.'s initial response to the Opinion has not been positive.See the statement by the Minister of State for Foreign Affairs, House of Commons, from February 26, 2019. It is notable that in G.A. Res. 68/262 (Mar. 27, 2014), the Assembly sough to uphold Ukraine's territorial integrity by calling on all states not to recognize Crimea's illegal declaration of independence.