Mauritius v. UK: Chagos Marine Protected Area Unlawful

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On 1 April 2010, the UK declared the world’s largest Marine Protected Area (MPA) around the Chagos Archipelago. The Archipelago is one of 14 remaining British overseas territories, administered by the UK as the British Indian Ocean Territory (BIOT). In contrast to other British overseas territories such as the Falklands/Malvinas and Gibraltar, BIOT is not on the UN list of non-self-governing territories. There is currently no permanent local population because the UK cleared the archipelago of the Chagossians between 1968 and 1973.

Mauritius and the UK both claim sovereignty over the Chagos Archipelago. The largest island of the Chagos Archipelago – Diego Garcia – has since the late 1960s housed the most important US military base in the Indian Ocean. The UK leased the island for defense purposes to the US in 1966, prior to Mauritian independence in 1968. The 50-year lease of Diego Garcia is due to be renewed in 2016.

In the Matter of the Chagos Marine Protected Area Arbitration (Mauritius v. UK), a tribunal constituted under Annex VII of the UN Convention on the Law of the Sea (UNCLOS) issued its award on 18 March 2015. The tribunal found that the UK’s declaration of the MPA disregarded Mauritius’ rights, rendering the MPA unlawful. The award raises the prospect that the renewal of the lease in 2016 will require the UK to meaningfully consult Mauritius.

Mauritius made four submissions to the tribunal:

First: The UK was not entitled to declare a MPA because it was not a coastal state under UNCLOS (the ‘sovereignty claim’, according to the UK)

Second: The UK was prevented from unilaterally declaring the MPA due to Mauritius’ rights as a coastal state under UNCLOS

Third: The UK may not take any steps to prevent the Commission on the Limits of the Continental Shelf from making recommendations to Mauritius in respect of any full submission to the Commission that Mauritius may make

Fourth: The UK’s declaration of the MPA was incompatible with substantive and procedural obligations under UNCLOS

The jurisdictional part of the award is centered on whether the four submissions concern the ‘interpretation or application of UNCLOS’ under Article 288 UNCLOS. This blog entry concentrates on the merits as regards the Fourth Submission.

The tribunal found that it only had jurisdiction with respect to Mauritius’ Fourth Submission, specifically its claims under Articles 2(3), 56 (2), 194 and 300 UNCLOS. The Dissenting and Concurring Opinion by Judges Kateka and Wolfrum disagreed. In their view, the dispute pertaining to the first two submissions was not merely an artificial re-characterization of the sovereignty dispute, but directly implicated UNCLOS.

The Lancaster House Undertakings as Binding Commitments

In 1965, Mauritius and the UK negotiated in London on independence. Under the so-called Lancaster House Undertakings, the UK made several commitments on Mauritian independence, including with respect to fishing rights in the waters surrounding Chagos, mineral and oil rights in the seabed and subsoil, and a reversionary interest of Mauritius in the archipelago once it was no longer needed for defense purposes. At the same time, the UK detached the Chagos Archipelago from Mauritius – formally with the agreement of the Mauritian Council of Ministers headed by a British governor – even though it had administratively formed part of Mauritius when it was a British colony.

The tribunal first examines what rights Mauritius had with respect to Chagos, in particular by examining the Lancaster House Undertakings. The tribunal found that ‘the undertakings provided by the United Kingdom at Lancaster House formed part of the quid pro quo through which Mauritian agreement to the detachment of the Chagos Archipelago from Mauritius was procured’ (para. 421). The Mauritian representatives modified their negotiation position in crucial respects in response to these undertakings. In the tribunal’s view, they were a sine qua none for Mauritius’ consent to detachment. The UK itself used the language of obligations to describe these undertakings designed to shape the relations between itself and Mauritius following independence, and nothing in the record indicated that the UK did not intend to be bound by these undertakings.

The tribunal accepted the UK’s argument that prior to independence, the agreement between the UK and Mauritius was a matter for British constitutional law alone. Nevertheless, it held that Mauritius’ independence in 1968 ‘elevat[ed] the package deal … to the international plane and transform[ed] the commitments made in 1965 into an international agreement’ (para. 425). As a result, they had become binding on the UK as a matter of international law.


The UK’s frequent repetition of these undertakings since 1965 lent support to the tribunal’s conclusion that these undertakings were binding. The tribunal reviewed in detail correspondence between the parties that showed the UK reaffirmation of these undertakings. The tribunal noted that estoppel as a ‘general principle of law’ (para. 435), ‘does not distinguish between representations as to existing facts and those regarding promises of future action or declarations of law’ (para. 437).

Based on its review of the case law, the Tribunal established the following test for estoppel:

estoppel may be invoked where (a) a State has made clear and consistent representations, by word, conduct, or silence; (b) such representations were made through an agent authorized to speak for the State with respect to the matter in question; (c) the State invoking estoppel was induced by such representations to act to its detriment, to suffer a prejudice, or to convey a benefit upon the representing State; and (d) such reliance was legitimate, as the representation was one on which that State was entitled to rely.

The Tribunal concluded that the Lancaster House Undertakings satisfied all four limbs of this test. Specifically with regard to (c), the tribunal found that ‘Mauritius’ entire course of conduct with respect to the Chagos Archipelago was undertaken in reliance on the full package of undertakings given at Lancaster House …  In so relying, Mauritius forewent the opportunity of asserting its sovereignty claim more aggressively, in particular in the early years following independence.’ (para. 442). Turning to (d), the tribunal concluded that Mauritius could legitimately rely on the UK’s representations. It explained that estoppel comes into play in the ‘grey area of representations and commitments whose original legal intent may be ambiguous or obscure, but which, in light of the reliance placed upon them, warrant recognition in international law.’ (para. 446).

The scope of Mauritian rights and alleged breaches of UNCLOS

After having found that the UK was estopped from denying the benefit of the Lancaster House undertakings to Mauritius, the tribunal turned to the character and scope of the Mauritian rights. The tribunal’s starting point was the express words of the undertaking. With regard to fishing rights, the tribunal found that the UK was obliged to ensure that fishing rights ‘remain available’ to Mauritius subject to certain limitations (para. 453). Over decades, the UK acted consistently with this obligation by preferentially allowing Mauritian vessels to fish around the archipelago, including in the territorial sea, even when vessels with other flags enjoyed no such rights.

The tribunal next considered whether the UK’s declaration of the MPA on 1 April 2010 violated Mauritian rights under UNCLOS. Mauritius submitted that the declaration of the MPA, ignoring Mauritius’ repeated calls for bilateral consultations, without notice and with immediate effect, had extinguished its rights in the territorial sea, in breach of Art 2(3) UNCLOS. It also alleged that the UK as a coastal state failed to pay due regard to Mauritius’ rights in the EEZ under Art 56 (2) and failed to endeavor to harmonize policies on marine pollution under Art 194. Finally, Mauritius contended that the UK had an ulterior motive in enacting the MPA – namely the goal of preventing the Chagossians from returning to the archipelago – which constituted an abuse of rights under Art 300.

The Tribunal’s findings on the Fourth Submission

The tribunal found that the declaration of the MPA clearly affected Mauritius’ rights. The binding undertaking of returning the archipelago to Mauritius when no longer needed for defense purposes at Lancaster House gave ‘Mauritius an interest in significant decisions that bear upon its possible future use. The declaration of the MPA was such a decision and will invariably affect the state of the Archipelago when it is eventually returned to Mauritius’ (para. 521). The UK was bound, as a matter of good faith, to give due regard to Mauritian rights, but failed to do so. Furthermore, the consultations with Mauritius were inadequate.

The Tribunal cited the following factual elements in support of this conclusion: the UK raised the possible declaration of the MPA ‘only in the vaguest possible terms’ with Mauritius (para. 523) and provided virtually no information to Mauritius; bilateral consultations commenced only in July 2009 and remained superficial, even though extensive consultations with the US had started earlier and took US concerns about the effects of the MPA on Diego Garcia seriously; the UK created an expectation of genuine bilateral consultations (which never materialized) and assured Mauritius that no decision on the MPA had yet been taken, only to announce the creation of the MPA a few days later; it failed to provide a convincing explanation for the urgency of declaring the MPA.

The tribunal found that the UK’s ‘approach to consultations with the United States provides a practical example of due regard and a yardstick against which communications with Mauritius can be measured’ (para. 528). The bilateral consultations with Mauritius, however, fell considerably short of this yardstick, reminding the tribunal of ‘ships passing in the night, in which neither side fully engaged with the other regarding fishing rights or the proposal for the MPA’ (para. 521). There was little evidence of a ‘spirit of negotiation and consultation’ or of a ‘balanc[ing] of interests at stake’ (para. 531). Judged against the US yardstick, the tribunal would have expected ‘significant further engagement with Mauritius’ (para. 533). It finally noted: ‘To the extent that the timing of the declaration of the MPA was in fact dictated by the electoral timetable in the United Kingdom or an anticipated change of government, the Tribunal does not accept that such considerations can justify the disregard of the United Kingdom’s obligations to Mauritius.’ (para. 533).

In sum, the UK failed to balance its own rights and interests with those of Mauritius. It simply assumed that the MPA was in the interest of Mauritius – without ever asking Mauritius – in clear departure from the intensive consultations with the US. Finding a breach of Art 2(3) and 56(2) and Art 194 (4) UNCLOS, the tribunal concluded the creation of the MPA was unlawful. The tribunal underscored that it expects the UK and Mauritius to enter into negotiations on protecting the marine environment in the Chagos Archipelago, without prejudice to their long-standing sovereignty dispute.

Concurrent and Dissenting Opinion

The partial dissenting opinion would have upheld the Tribunal’s jurisdiction over the first and second submission. Judges Kateka and Wolfrum regretted that the Tribunal failed to examine the legality of the detachment of Chagos from Mauritius prior to independence in light of self-determination and uti possidetis. In their view, UK Prime Minister Harold Wilson’s threat to Mauritian Prime Minister Ramgoolam that he had the ‘choice’ of returning home to Mauritius without independence amounted to a duress (para. 77). As a result, Mauritius did not validly consent to the detachment of its archipelago.

In addition, the UK did not act in good faith in declaring the MPA (para. 90). Judges Kateka and Wolfrum note

‘disturbing similarities between the establishment of BIOT in 1965 and the establishment of the MPA in 2010. Although these events are 45 years apart, they show a certain pattern. This is the disregard of the rights and interests of Mauritius. The 1965 excision of the Chagos Archipelago from Mauritius shows a complete disregard for the territorial integrity of Mauritius by the United Kingdom which was the colonial power. British and American defence interests were put above Mauritius’ rights. Fast forward to 2010 and one finds a similar disregard of Mauritius’ rights, such as the total ban on fishing in the MPA. These are not accidental happenings (para. 91).


The award by the distinguished Tribunal has numerous nuggets that will make it required reading for courses on the law of the sea and the settlement of international disputes. The award sheds important light on the scope of compulsory dispute settlement under UNCLOS, and reveals an important disagreement between the majority and minority on the scope of dispute settlement under Art 288 UNCLOS. The Tribunal’s objective qualification of the UK’s undertakings and the use of estoppel in this context could have important implications for other sovereignty disputes in a decolonization context.

Even if the tribunal upheld only one of Mauritius’s four claims, the award plus the dissent could help mobilize opinion in favour of a negotiated settlement on the Chagos Archipelago, including the return of the Chagossians. The arbitration itself put the UK in the awkward position of having to defend a colonial legacy in the Indian Ocean whose establishment is at least in tension with the legal principles applicable to decolonization. The detailed submissions by both parties (including many original documents) will allow students and others to judge for themselves whether the UK’s actions in 1965 conformed to these principles.

More immediately, the UK will find it difficult to avoid engaging in serious consultations with Mauritius on the renewal of the lease of Diego Garcia to the United States in 2016. The renewal is likely as significant for the condition in which the archipelago will eventually return to Mauritius as the MPA which the Tribunal considered to be unlawful. Mauritius will likely insist that the UK recognize it for the first time as an equal partner in these discussions. Likewise, the award keeps the issue of the return of the Chagossians to the archipelago high on the agenda.

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