Source: US Global Legal Monitor

The following is a guest post by Clare Feikert-Ahalt, a senior foreign law specialist at the Law Library of Congress covering the United Kingdom and several other jurisdictions. Clare has written numerous posts for In Custodia Legis, including Revealing the Presence of Ghosts; Weird Laws, or Urban Legends?; FALQs: Brexit Referendum; 100 Years of “Poppy Day” in the United Kingdom; and Mr. Bates vs. The Post Office Spurs Possible Law Change.

Yesterday’s post described the historic status of the Chagos Archipelago and the United Kingdom’s (UK) power over the territory. Today’s post describes the new agreement, which returns sovereignty over the Chagos Archipelago and allows for the continued use of the UK-US military base.

On May 22, 2025, the United Kingdom and Mauritius signed an agreement that “recognis[es] the wrongs of the past” with regards to the Chagos Archipelago. The agreement transfers sovereignty of the British Indian Ocean Territory (BIOT) from the UK to Mauritius, while providing the UK with “rights and authorities [over Diego Garcia] that the United Kingdom requires for the long-term, secure and effective operation of the Base.”

The agreement, which took over two years and 13 rounds of negotiations to achieve, secures British interests in Diego Garcia, including an area of 12 nautical miles surrounding the island, for 99 years. The agreement provides the UK with the right to access, maintain, and invest in the base, along with the ability to use it for defense purposes. It places a binding obligation on both parties to ensure the secure and effective operation of the base. The UK’s secretary of state for defence notes the agreement achieves the “secured unrestricted access to, and use of, the base, as well as control over movement of all persons and all goods on the base and control of all communication and electronic systems.”

Any activities on the wider islands of the Chagos Archipelago, such as the construction of any structure, artificial island, sensor, or barrier within 24 nautical miles, must be approved through a joint decision process between the UK and Mauritius, which serves as an “effective veto” of development in the islands surrounding Diego Garcia as the UK does not want other countries, particularly those hostile to the UK, to have a presence near this facility.

The 99 years can be extended for a further 40 years if both parties agree, and it may be extended again thereafter. The estimated cost to UK for 99 years “is £101 million [annually] and the net present value of payments under the treaty is £3.4 billion” (approximately US$136 million and US$4.6 billion respectively) accounting for approximately 0.2% of the defense budget. The government has stated this is less than the cost of running an aircraft carrier, without aircraft, for a year.

The agreement provides for the resettlement of the residents of Diego Garcia, known as the Chagossians, on the islands of the Chagos Archipelago, with the exception of Diego Garcia. It also provides for the establishment of a trust fund of £40 million (approximately US$54 million) to benefit Chagossians and an annual grant of £45 million (approximately US$61 million) for 25 years to fund projects that promote economic development and welfare in Mauritius. Article 11 of the agreement states that it “constitutes the full and final settlement of all claims by Mauritius in relation to the Chagos Archipelago.”

The treaty was laid before both Houses of Parliament on May 22, 2025, and either of the Houses of Parliament may object to its ratification until July 3, 3035.

The Defense Facility on Diego Garcia

The secretary of state for defence for the UK stated “[t]he importance of Diego Garcia cannot be overstated” and a government press release announcing the agreement notes that the base is central to both the UK and US’s emergency planning and operations, with the base serving as:

“a critical logistics hub at a strategic location, with a full range of facilities that acts as a key refueling and resupply station for naval and air operations. This enables power projection and global reach, allowing for rapid and flexible deployment of our forces across the Middle East, East Africa, and South Asia.”

While most of the work on, and capabilities of, Diego Garcia are not disclosed, the secretary of state for defence and the UK prime minister have publicly acknowledged that the base supports operations, including those related to counter-terrorism, in the Middle East, East Africa, and South Asia. Public statements detail that the base houses:

• an airfield enabling strike operations and the rapid deployment of the military in this area, “… creat[ing] real military advantage across the Indo-Pacific;”
• a deep-water port that, among other uses, “supports missions from nuclear-powered submarines to [the UK’s] carrier strike group;”
• advanced communications, which includes management of the electromagnetic spectrum satellite;
• surveillance capabilities;
• facilities that support the global operation of GPS, notably one monitoring station and one of four ground antennas;
• Ground-Base Electro-Optical Deep Space Surveillance (GEODSS) System, which “provides situational awareness of objects in Earth’s orbit, helping to track space debris that pose a risk to space systems”; and
• “three pieces of critical Comprehensive Nuclear Test Ban Treaty monitoring equipment”, including seismic monitoring equipment that checks for indicators of nuclear testing, helping to secure compliance with the nuclear test ban treaty.

The presence of the base in the center of the Indian Ocean also helps to safeguard an important trade route, through which “a third of the world’s bulk cargo and two-thirds of global oil shipments are transported.”

The US Navy describes the facility on Diego Garcia as “the tip of the spear” and states that it “provides logistic support to operational forces forward deployed to the Indian Ocean and Persian Gulf areas of responsibility in support of national policy objectives.”

The prime minister stated that the agreement is vital to the UK’s defence and intelligence, and for securing the safety and security of the British people at this time. He stated “… the base was under threat” from legal challenges by Mauritius, and the government believes there is no viable alternative to protect the base and secure the islands surrounding it.

The prime minister further noted that if the UK disregarded any future legal judgements, “international organisations and other countries would act on them. And that would undermine the operation of the base.” The UK was particularly concerned at the prospect of other countries establishing a presence in the islands surrounding Diego Garcia, or conducting training exercises nearby, which could impact the operation of the base, and that it would be unable to prevent this without an agreement.

The prime minister has described the base as “one of the most significant contributions we make to our security relationship with the United States.” The UK foreign secretary stated the US was unhappy with the uncertainty created by the situation and “strongly encouraged [the UK] to strike a deal.” It was against this background that negotiations were commenced and the treaty was made.

Reaction to the Agreement

The opposition conservative party has been critical of the agreement, stating that the government “prioritised heeding the most pessimistic legal advice” concerning the potential of legal judgments. The opposition further stated that the agreement puts the defense facility at risk due to Mauritius’ ties to Russia and China. The UK shadow secretary of state said in parliament that “[t]he Government should not be surrendering strategically vital sovereign territory, especially when we face such threats, and they certainly should not be paying billions for the privilege”, noting further that the agreement does not offer any protection to the Chagossians.

Internationally, the agreement has been backed by the UK’s “Five Eyes” partners, which include the United States, Canada, Australia, and New Zealand. Japan, India, and the African Union have also welcomed the agreement. US President Donald Trump expressed his support for the agreement and US Secretary of State Marco Rubio, stated that while the administration is not a party to the agreement, it “remain[s] responsible for operating the U.S. Naval Support Facility on Diego Garcia, which continues to play a vital role in supporting forward-deployed operational forces and advancing security across the region.”

The US secretary of state stated:

“The Trump Administration determined that this agreement secures the long-term, stable, and effective operation of the joint U.S.-UK military facility at Diego Garcia. This is a critical asset for regional and global security.”

While the agreement has been welcomed by the UK and several of its allies, the United Nations has condemned the agreement, issuing a press release stating:

“By maintaining a foreign military presence of the United Kingdom and the United States on Diego Garcia and preventing the Chagossian people from returning to Diego Garcia, the agreement appears to be at variance with the Chagossians’ right to return, which also hinders their ability to exercise their cultural rights in accessing their ancestral lands from which they were expelled.”

The UN has urged the UK to “apply a human rights-based approach in addressing historical injustices against the Chagossian people.”

Additional Law Library of Congress Resources on the Laws of Mauritius and the UK

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