The UK government has delayed further debate in the House of Lords on ratification of the Chagos Archipelago treaty, an agreement signed by the UK and Mauritius in May 2025. The pause follows an amendment tabled by the UK Conservative Party asserting that completion of the ratification process would breach a 1966 agreement between the US and UK regarding the joint military bases in the archipelago.
The 2025 Chagos Archipelago treaty between Mauritius and the UK would transfer sovereignty of the islands, including the large military base on Diego Garcia that is shared with the US, to Mauritius. However, at the same time, the UK would obtain a 99-year lease over Diego Garcia, which would assure that the base there would continue to operate as before. That lease can be extended for further periods.
The history of Mauritian independence and the Chagos Islands
Mauritius was a French colonial territory until 1814, when it was ceded to the UK after the defeat of Napoleon in the Treaty of Paris. The Chagos Archipelago fell within the colonial administration of Mauritius.
However, the UK has attempted to argue that it was administered to some extent separately. This matters, because under the international law doctrine of ‘uti possidetis’, a colony has the right to gain independence for all territory administered as one colony by another country.
But at the height of the Cold War, as Mauritian independence loomed, Diego Garcia, the largest island in the archipelago, was found to be best placed to host a major US and UK military base.
In 1965, the UK convened a conference at Lancaster House in London, where representatives from Mauritius agreed to the excision of the Chagos Archipelago from the remainder of the colony in exchange for a payment of £3 million. The population of the islands was to be largely removed.
That same year, the UK transformed the archipelago into the separate legal entity of the British Indian Ocean Territory (BIOT), which was henceforth excluded from the Mauritian decolonization process. It was accordingly not included in the territorial definition of Mauritius when it gained independence in 1968.
After independence, Mauritius began campaigning for the return of the archipelago with the backing of the United Nations (UN). In 2017, the UN General Assembly requested an advisory opinion from the International Court of Justice (ICJ) on the legal situation and its consequences.
Mauritius argued that its process of decolonization had not been completed, pending the handover of the archipelago. The UK, somewhat dubiously, asserted that the principle of self-determination only turned into a firm right of colonial peoples by 1970, i.e. after the excision of the BIOT, to which the Mauritius authorities had in any event agreed.
In 2019, the ICJ advised that the excision of the Chagos Archipelago had been unlawful. The Court found that the consent, supposedly given at the Lancaster House Conference, was not valid as the pre-independence authorities of Mauritius represented at the conference were still dependent on the UK.
Moreover, the people concerned had not been given the opportunity to give full and informed consent, as is required by the doctrine of self-determination. The ICJ found that the UK was committing an ongoing violation of international law and confirmed London’s obligation to end its administration of the archipelago as rapidly as possible.
An ICJ advisory opinion is not legally binding. However, it represents a highly authoritative rendering of the law. The findings of the Court were endorsed by the UN General Assembly, in Resolution 73/295. Other courts and tribunals have already based their decisions on the ruling of the ICJ and some of these are, in turn, legally binding. Moreover, the UK relies on its reputation as a brand leader where the international rule of law is concerned.
The then Conservative UK government accordingly commenced negotiations on the return of the archipelago, which were completed by the present Labour government. As the deal took shape, some opposition emerged to the handover, mainly for geostrategic reasons.
US views on the proposed deal were considered critical: Initial indications suggested that the new, second Trump administration would oppose it. However, by February 2025, it seemed the US administration was content with the arrangement, which would safeguard its base for at least 99 years. The deal was therefore signed.
The UK delay
Now, attention has been drawn to the agreement concluded between the US and UK in 1966, just after the excision of the Chagos Islands and before the independence of Mauritius.
That agreement granted extensive basing rights to the US in the archipelago and would run for an initial period of 50 years, extending itself automatically thereafter for a further period of 20 years, to 2036.
The very first clause of the 1966 agreement states that ‘The territory shall remain under United Kingdom sovereignty’. This reads as if the UK promises to the US that the archipelago shall forever be British – an undertaking that would be broken by ratification of the 2025 agreement.
However, this would be a misreading of the intent of the parties to the US–UK agreement and disregard the context in which that statement is made. The purpose of this paragraph was to re-assure the UK, not the US. It confirms that, despite the broad rights granted to the US in the agreement, it would be Britain that retained sovereignty. This was no oath of eternal UK sovereignty over the Chagos Archipelago, or Diego Garcia.
True, the agreement refers to the possibility that the archipelago remains available to meet defence needs for an ‘indefinitely long period.’ However, this is no substantive obligation, but merely something for the two governments to ‘contemplate.’
Instead, that very paragraph addresses the expiry of the agreement, and states that it can be terminated by just one side with two years notice. This removes any suggestion that the UK has contracted into an eternal obligation. Eternity is not subject to expiry or termination.
International law, instead, recognizes that circumstances can change. Where intervening circumstances have radically transformed the balance of obligations of the sides, a treaty may be impeached or changed. In this instance, the higher order legal obligation of self-determination and territorial unity for Mauritius has intervened, forcing an adjustment of the situation.
When balanced against the fact that the UK is committing a serious and ongoing violation of international law by retaining sovereignty, the change in status of Diego Garcia loses relevance. That is reinforced by the fact that the UK has negotiated an outcome that avoids any adverse effect on US base operations, which the 1966 treaty was meant to safeguard.
