The right to license lucrative online domain names favoured by cryptocurrency firms has become the latest battlefield in the escalating dispute between Mauritius and the UK over ownership of the Chagos Islands. The two letter address .io, which stands for Indian Ocean, is allocated by the Internet Assigned Numbers Authority (Iana) to the British Indian Ocean Territory (BIOT). It is operated by a commercial firm nominally based in Diego Garcia, the largest of the Chagos islands and home to a US military base. The .io term is fashionable among online dealers and digital economy enthusiasts because the initials also stand for input/output, the process of sending and receiving data. Cryptocurrency firms and companies selling non-fungible tokens (NFTs) – created to turn digital art into tradeable videos or images – frequently select .io web addresses for their trading platforms. They are consequently more expensive. Both the Mauritian government and exiled Chagossians – forcibly removed by the UK from their homes in the early 1970s – are challenging British control over .io domain names on the grounds that Britain no longer has a legitimate claim to the Chagos Islands. The legal balance in favour of Mauritian arguments for sovereignty shifted decisively after decisions at the international court of justice (ICJ), the UN general assembly and the international tribunal of the law of the sea (Itlos) in 2019 and last year. Britain was found to have unlawfully separated the Chagos Islands from Mauritius before granting independence in 1968. The UK is defying UN votes and court judgments demanding the return of the islands, stressing that the ICJ ruling was only an “advisory opinion”. Legal advice drafted in New York for Mauritius – and seen by the Guardian – shows that the government in Port Louis is considering how to enforce its online authority. It comes after after an expedition last month to raise its flag over the outer islands in the archipelago. Mauritius’ strategic objectives are to “end the use of .io domain name by the United Kingdom or any entity acting on its behalf and to secure for the government of Mauritius revenues generated by any further use of the .io domain name”. Iana lists the Internet Computer Bureau (ICB) Ltd as the company that allocates the country code for .io domain names. The firm also controls sales of .ac domains related to part of another British overseas territory, the Ascension Islands. The Iana website records ICB’s address as: c/o Sure (Diego Garcia) Limited, Diego Garcia, British Indian Ocean Territories, PSC 466 Box 59 FPO-AP 96595-0059, British Indian Ocean Territory. The firm was sold in 2017, reportedly for $70m (£53m). It has been bought again since then. The previous owner of ICB said in 2014 that profits from selling .io domain names are “distributed to the [British] authorities”. According to the Foreign, Commonwealth and Development Office, however, neither BIOT nor the UK government benefit from sales of .io domain names. A spokesperson said: “The UK has no doubt as to our sovereignty over the British Indian Ocean Territory, which we have held continuously since 1814. Mauritius has never held sovereignty over the territory and the UK does not recognise its claim.” Jagdish Koonjul, the Mauritian ambassador to the UN, said his government had contacted ICB through its Diego Garcia address asking them to deal in future with the Mauritian government. “We have written to Sure [in Diego Garcia] but they have only said they are looking at it,” he said. In a separate initiative, lawyers for the Chagos Refugees Group have also lodged a complaint about British control of the .io domain name with the OECD (Organisation for Economic Cooperation and Development). The ICB and its parent company were approached for comment.