In May 2025, I authored an article entitled Chagos Deal Is Done: Sovereignty Is Returned to Mauritius. Arguably, that title was somewhat premature, as a great deal has happened since then. The past eight months have seen the legislation required to return the Chagos Archipelago to Mauritius progress through most of its parliamentary stages (though not without difficulty), opponents intensifying their objections, and signs of a possible change of heart by the Trump administration, which had previously been supportive. Yet although much has changed—at least rhetorically—the fundamentals of the case for returning Chagos to Mauritius remain sound.
The Diego Garcia Military Base and British Indian Ocean Territory Bill passed Second Reading on 9 September and completed Committee of the whole House and Remaining Stages on 20 October, with MPs voting 318–174 to retain the central clause formally ceding UK sovereignty over the British Indian Ocean Territory (BIOT)—the UK’s official designation for the Chagos Archipelago. The Bill then encountered closer scrutiny in the House of Lords. At Report Stage in early January 2026, peers added amendments aimed at increasing financial transparency, creating a mechanism to pause or halt payments if the base became unusable or if Mauritius failed to meet obligations following dispute‑resolution procedures, and requiring a referendum of Chagossians on the treaty’s terms. The changes were subsequently overturned in the Commons, with Ministers arguing that the treaty already included robust dispute‑settlement provisions and that a referendum cannot be used to reopen a concluded state‑to‑state agreement.
As debate intensified in late January, a final planned Lords sitting was paused—and remains so—partly because opposition peers linked the Bill’s timing to concerns about potential friction with the 1966 US–UK agreement governing defence use of BIOT. Opponents have cited Article 1, which states that “The Territory shall remain under United Kingdom sovereignty”, to argue that the UK cannot ratify a treaty recognising Mauritian sovereignty without first amending the 1966 framework. However, it does appear these concerns have been overstated. First, the 1966 document is an ‘exchange of notes’ and not a full treaty, meaning it is less formal. Second, it has been argued that the agreement was designed to allow the functioning of the joint military base on Diego Garcia during the Cold War, not to freeze sovereignty arrangements permanently. The 2025 treaty maintains uninterrupted US–UK basing, preserving all operational guarantees while transferring sovereignty to Mauritius. Therefore, ratification would not violate international law or undermine defence commitments, and the 1966 agreement poses no legal barrier.
The cost of the deal has been another flashpoint, but again there has been some disinformation. Critics cite a headline £35 billion figure, which simply totals 99 years of payments without accounting for inflation or the time value of money. However, government calculations apply the Treasury’s social time preference rate, discounting future payments by 2.5–3.5% annually, producing a net present value of about £3.4 billion. This discounted figure is more economically meaningful because it reflects what the payments are worth in today’s money rather than their inflated nominal sum over a century-long horizon. It is still a significant amount, although equivalent to 0.2% of the UK’s annual defence budget.
The path towards ratification appeared to become more difficult when US President Donald Trump denounced the agreement as an “act of GREAT STUPIDITY”, linking it rhetorically to his push to acquire Greenland and accusing the UK of handing over vital territory “for no reason whatsoever”. This public reversal surprised London, given that the US had explicitly backed the deal in May 2025, with Secretary of State Marco Rubio stating that it “secures the long‑term, stable, and effective operation of the joint US–UK military facility at Diego Garcia”. UK ministers continue to say that relevant US departments and intelligence agencies stand by that earlier assessment.
Trump’s comments created political ripples, emboldening opponents to intensify their lobbying efforts in Washington. This included a newly energised ‘Chagos government in exile’, whose delegation reportedly offered to rename one of the Eagle Islands in the Chagos Archipelago ‘Trump Island’ if the US President moved to block the deal. Meanwhile, much of the press continued to accuse the UK government of “surrender” in negotiating with Mauritius. Yet the deal that Rubio and other US officials supported remains unchanged. The operative clauses of the 2025 treaty largely mirror longstanding practice: the UK and US retain “unrestricted access” to Diego Garcia’s air and sea space for defence; the UK maintains jurisdiction and control over personnel on the base; Mauritius is notified after, rather than asked to consent before, any armed action; and stringent security‑review vetoes continue to govern installations near the island. The treaty’s annexes further codify and strengthen these protections. Beyond Diego Garcia itself, any proposal involving third‑country security forces or new construction within 12–24 nautical miles triggers mandatory joint security reviews.
These safeguards underpin the support expressed by allies—including all Five Eyes partners and regional actors such as India and Japan—alongside the earlier US endorsement. They also strengthen many analysts’ assessments that the treaty reduces, rather than expands, opportunities for adversary influence in the central Indian Ocean. International legal considerations—often overshadowed in recent months—remain pivotal. In 2019, the International Court of Justice advised that the detachment of Chagos from Mauritius in 1965 was unlawful and that the UK should end its administration “as rapidly as possible”. The UN General Assembly endorsed that view, and the UN system has since behaved as if Mauritius is the relevant coastal state. For example, the 2021 International Tribunal for the Law of the Sea judgment in the Mauritius–Maldives maritime case proceeded on that basis.
Specialised bodies—including the Universal Postal Union and elements of nuclear test‑ban and telecommunications governance—have also shown a willingness to treat UK legal claims over BIOT as contested or inapplicable. This raises practical risks for communications, overflights, and contractor operations essential to the base. Without the deal, a new round of contentious proceedings could quickly have led to provisional measures or institutional decisions that—regardless of UK acceptance of jurisdiction—would impede aspects of Diego Garcia’s functioning.
Despite the turbulence of recent months—parliamentary delays, sharpened domestic opposition, and shifting rhetoric from Washington—the fundamental case for the UK–Mauritius agreement remains intact. The legal, strategic, and geopolitical drivers underpinning the 2025 treaty have not changed. If anything, debates in late 2025 and early 2026 have highlighted how essential a negotiated settlement is to stabilising the UK’s position, ensuring operational certainty for the base, and preventing further international legal erosion of British (and, by extension, US) claims over BIOT.
Where the Government has struggled is not in the substance of the deal but in its political stewardship. As in other major policy areas, Keir Starmer and the Labour Government have not consistently been proactive in explaining, defending, and contextualising the agreement. Critics—both principled and opportunistic—have therefore filled the political space, framing the treaty as a strategic concession rather than a pragmatic solution to structural legal vulnerabilities the UK could no longer ignore. Yet the treaty itself has not shifted in any way that justifies this backlash: the operational guarantees for the UK and US remain robust, and the security architecture around Diego Garcia is arguably stronger than before.
For all the noise, the deal remains likely to pass. Parliamentary arithmetic favours approval once the Commons reasserts itself, and allies—especially Five Eyes partners—continue to regard the treaty as stabilising. Even if President Trump moved from rhetorical opposition to concrete action to block implementation, this would not settle the matter – it would merely defer it. The underlying international legal issues would immediately reassert themselves and Mauritius would resume contentious proceedings at the ICJ, before ITLOS, and across multiple specialised UN agencies. Each round of litigation or institutional decision‑making would further erode the UK/US position, increasing operational uncertainty around Diego Garcia and risking precisely the kind of disruption opponents of the treaty claim to fear.
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