India has consistently supported Mauritius’s claim of sovereignty over Chagos Islands, in line with its “principled stand on decolonisation and support for the sovereignty and territorial integrity of nations”, the external affairs ministry said after the UK and Mauritius jointly announced the agreement.

However, the military base at Diego Garcia allows the US Navy to maintain an active presence in the Indian Ocean at a time when the Chinese Navy has significantly stepped up its activities in the region, people familiar with the matter said on condition of anonymity.

“It is believed the final outcome is a win for all sides involved and will reinforce long-term security in the Indian Ocean region,” one of the people said.

The deal grants Mauritius full sovereignty over the archipelago, while simultaneously guaranteeing the operation of the US-UK military base for 99 years for maintaining regional stability and international security.

The political agreement is subject to a treaty and supporting legal instruments being finalised, the British government said in a statement. “Both sides have committed to complete this as quickly as possible,” it added.

Britain controlled the region since 1814 and it detached the Chagos archipelago from its former colony of Mauritius in 1965 to create the British Indian Ocean Territory (BIOT). In 1966, it leased Diego Garcia, the largest island in the archipelago, to the US. About three years after Mauritius became independent in 1968, Britain evicted nearly 2,000 islanders to Mauritius and the Seychelles to make way for a US naval support facility on Diego Garcia.

The British statement said the deal with Mauritius will protect national security by “safeguarding the operation of the strategic military base”.

The deal “sees Mauritius assume sovereignty over BIOT, with the UK authorised to exercise the sovereign rights of Mauritius on Diego Garcia”, the statement said. The agreement also reflects the UK government’s commitment to “safeguarding global security and averting threats to peace and prosperity in the Indian Ocean and wider Indo-Pacific”.

India welcomed the UK-Mauritius agreement on the “return of Mauritian sovereignty over the Chagos archipelago, including Diego Garcia” as it “completes the decolonisation of Mauritius” after two years of negotiations, the external affairs ministry said.

India is committed to working with Mauritius and other like-minded partners in strengthening maritime security and contributing to peace and prosperity in the Indian Ocean, the ministry added.

The people cited above said the Indian side played a “quiet but important role in the background” during the negotiations. India firmly backed the Mauritian position, supporting its stance on the need to do away with the last vestiges of decolonisation, while simultaneously encouraging both sides to negotiate with an open mind and with a “view to achieving mutually beneficial outcomes”, the people said.

The UK government described the deal with Mauritius as historic and said: “For the first time in more than 50 years, the status of the [Diego Garcia] base will be undisputed and legally secure”.

The operation of the military base will remain “unchanged, in an increasingly volatile world”, it said. The agreement is strongly supported by the UK’s international partners including the US, it added.

Without the agreement, the long-term, secure operation of the military base would be “under threat, with contested sovereignty and legal challenges”, the UK government said.

Britain’s foreign secretary David Lammy said: “Today’s agreement secures this vital military base for the future. It will strengthen our role in safeguarding global security, shut down any possibility of the Indian Ocean being used as a dangerous illegal migration route to the UK, as well as guaranteeing our long-term relationship with Mauritius, a close Commonwealth partner.”

British high commissioner Lindy Cameron added that the agreement will enable “deeper cooperation with India in the Indian Ocean”.

Diego Garcia has seen a small number of vulnerable migrants arrive since 2021, and they subsequently made asylum claims with the UK. The agreement will “shut down any possibility of the Indian Ocean being used as a dangerous illegal migration route to the UK”, and Mauritius will take responsibility for any future arrivals, the British government said.

US President Joe Biden too welcomed the deal, saying in a statement that the US-UK military facility at Diego Garcia “plays a vital role in national, regional, and global security”.

“It enables the United States to support operations that demonstrate our shared commitment to regional stability, provide rapid response to crises, and counter some of the most challenging security threats we face. The agreement secures the effective operation of the joint facility on Diego Garcia into the next century,” Biden said.

The agreement is also a “clear demonstration that through diplomacy and partnership, countries can overcome long-standing historical challenges to reach peaceful and mutually beneficial outcomes”, Biden added.

US secretary of state Antony Blinken said the Biden administration strongly supported the negotiations between the UK and Mauritius over the past two years, and the agreement reflects “our shared commitment to a free and open Indo-Pacific region”.

Diego Garcia’s strategic location allows the US and UK militaries to quickly access East Africa, West Asia and South and Southeast Asia. The military base serves as a staging area for the build-up or resupply of military forces. The base has been used by the US during the operation to counter the Iraqi invasion of Kuwait in 1990 and for operations in Iraq and Afghanistan over the past three decades.