Jurisdiction overviewTax in British Indian Ocean Territory
Last reviewed: · by TaxProsRated editorial
Key points
British Indian Ocean Territory (BIOT) is a UK Overseas Territory in the Indian Ocean (Chagos Archipelago). There is no civilian population and no domestic tax framework. Approximately 3,000–4,000 UK and US military personnel rotate through Diego Garcia base. UK Armed Forces personnel are taxed under UK HMRC rules; US Armed Forces personnel under US IRS rules via Status of Forces Agreement. In October 2024 the UK signed a treaty transferring sovereignty of the Chagos Archipelago to Mauritius, while retaining Diego Garcia base under a 99-year lease. The transition is ongoing as of 2026.
No residents means no domestic tax system
Tax systems exist to collect revenue from residents and businesses. BIOT has neither. The entire archipelago is administered as a military support zone. Personnel are on rotation, not permanent residency. No domestic income tax, no corporate tax, no VAT, and no local currency in general circulation.
Who administers British Indian Ocean Territory?
BIOT is administered by the BIOT Administration, a unit within the UK's Foreign, Commonwealth and Development Office (FCDO) based in London. The Commissioner for BIOT is a senior FCDO official appointed by the Crown.
The legal foundation is the BIOT Constitution Order 2004 (and predecessor Orders in Council dating to 1965). UK law applies to the territory by Order in Council; there is no elected legislature and no resident civilian government.
For tax purposes there is no BIOT authority. No tax office, no filing system, and no domestic tax legislation specific to the islands exist. Personnel on Diego Garcia report to their home-country tax administrations: HMRC for UK Armed Forces, and the IRS for US Armed Forces.
The October 2024 UK–Mauritius sovereignty transfer
On 3 October 2024, the UK and Mauritius signed a treaty agreeing to transfer sovereignty of the Chagos Archipelago to Mauritius. This is the most significant constitutional development for BIOT since its creation in 1965.
- UK agrees to transfer sovereignty of all Chagos Archipelago islands to Mauritius
- UK retains Diego Garcia military base under a 99-year lease
- Mauritius receives financial compensation from the UK
- Transition is a formal treaty process — not yet complete as of 2026; ratification and implementation ongoing
- No change to civilian population status during transition: BIOT remains uninhabited by civilians
Once the treaty is fully ratified and implemented, the territory will cease to be a UK Overseas Territory. Mauritius will hold sovereignty, and the Mauritius Revenue Authority (MRA) will become the competent fiscal authority for any future civilian activity on non-base islands. Diego Garcia will continue to operate under UK and US military jurisdiction for the 99-year lease period.
The transition does not create any new filing obligations for personnel currently on Diego Garcia — military personnel continue to file with HMRC or IRS as applicable.
The Chagossian forced expulsion 1965–1973
BIOT's history includes a significant and contested human-rights episode. When the UK created BIOT in 1965 by detaching the Chagos Archipelago from then-Mauritius, approximately 1,500–2,000 indigenous Chagossians were forcibly relocated from the islands between 1965 and 1973 to make way for the Diego Garcia base.
- UN General Assembly Advisory Opinion (2017): called on the UK to end administration of BIOT
- International Court of Justice Advisory Opinion (February 2019): found the UK's continued administration of the Chagos Archipelago was not lawful under international law
- Ongoing displaced-persons litigation in UK courts by Chagossian diaspora communities
- October 2024 sovereignty treaty includes provisions acknowledging Chagossian right of abode on outer islands (not Diego Garcia base)
This historical context is factual and well-documented. It has no direct bearing on current tax obligations of personnel on Diego Garcia, but it is an important part of the territory's governance background.
Is there a BIOT filing calendar?
BIOT has no domestic filing calendar. There are no BIOT return dates, no BIOT assessment windows, and no BIOT tax forms.
UK Armed Forces personnel on Diego Garcia remain subject to HMRC's Self Assessment regime on the UK tax year (6 April to 5 April). US Armed Forces personnel follow the IRS calendar year (1 January to 31 December). Status of Forces Agreement (SOFA) rules govern jurisdiction for US personnel.
Is there BIOT tax residency?
No. There is no BIOT tax residency concept in any legal system. Physical presence on Diego Garcia does not create, suspend, or break tax residency under any country's domestic rules.
Personnel on Diego Garcia remain tax-resident in their home country throughout their assignment. Deployment to BIOT does not move a service member or civilian contractor off the HMRC or IRS residency register. The relevant question is always home-country residency status and any applicable SOFA provisions.
How is income from Diego Garcia service taxed?
Personnel on Diego Garcia are taxed entirely by their home-country tax authority. The source of payment — not the physical location of service — determines where income tax applies.
UK military and MOD civilian personnel on Diego Garcia are paid by the UK Crown. UK income tax, National Insurance, and PAYE apply under HMRC rules. Operational Service Relief may reduce income tax for qualifying deployed roles. See HMRC's guidance on armed forces pay.
US military and DoD civilian personnel operate under the Status of Forces Agreement (SOFA). US military pay is subject to US federal income tax. Combat zone tax exclusions under IRC §112 may apply for qualifying periods. SOFA prevents UK taxation of US personnel.
Third-country civilian contractors (non-UK, non-US) are taxed under their own home-country framework. No BIOT withholding or local filing requirement applies to anyone on the base.
The Diego Garcia US military base
Diego Garcia hosts one of the most strategically significant US military installations in the Indo-Pacific. The atoll serves as a logistics hub, naval support facility, and air base for US and UK operations across the Indian Ocean.
99-year UK lease retained post-2024 transfer
The October 2024 UK–Mauritius sovereignty treaty explicitly preserves the Diego Garcia base under a 99-year UK lease. US forces continue to operate under the existing 1966 US–UK defence agreement. Military jurisdiction on the atoll remains UK and US, regardless of which sovereign holds title to the broader archipelago. Post-transfer, Diego Garcia is functionally unchanged for its 3,000–4,000 rotating personnel.
Corporate and commercial activity
There are no companies incorporated in BIOT and no commercial operations based there outside of contracted base-support services. UK law formally extends to the territory by Order in Council.
UK Corporation Tax at 25% (from April 2023) formally applies to any commercial activity conducted under UK jurisdiction, including BIOT. No BIOT-specific corporate rate exists. In practice, no commercial entities are based in BIOT.
Once the 2024 treaty transition completes, Mauritius Revenue Authority (MRA) becomes the competent fiscal authority for non-base islands. Diego Garcia itself remains under UK–US jurisdiction per the 99-year lease. Mauritius CIT is 15%.
Indirect tax
There is no VAT, GST, or sales tax on BIOT. UK VAT at 20% is the formal reference framework, but no civilian transactions occur on the territory that would attract it.
| Scenario | Indirect-tax treatment |
|---|---|
| Provisions shipped to Diego Garcia for base operations | Export from UK; UK VAT zero-rated on export |
| US military equipment from Continental USA | SOFA provisions; US import rules |
| Base support contracts from UK-registered suppliers | UK VAT on UK supply; export zero-rating where applicable |
| Post-transfer (outer islands) | Mauritius VAT (15%) framework would govern future civilian activity |
The Diego Garcia Exclusive Economic Zone covers a significant area of the Indian Ocean. All maritime activity within that zone is subject to BIOT/UK jurisdiction — but no VAT or customs receipts flow to any BIOT authority. Fiscal obligations flow to UK HM Treasury.
Cryptoassets and digital assets
BIOT has no financial infrastructure and no digital-asset framework of its own. Cryptoasset activity by anyone on Diego Garcia is governed entirely by home-country rules.
All financial activity follows home-country rules
UK personnel: HMRC's crypto asset guidance (CRYPTO22100 series) applies — gains are Capital Gains Tax events reported on Self Assessment. US personnel: IRS Notice 2014-21 and subsequent guidance applies — crypto gains are taxable income. BIOT itself has no FCA, no SEC equivalent, and no financial regulator.
What is the treaty network?
BIOT has no bilateral double-tax agreements of its own. The UK's extensive DTA network formally covers territories within UK sovereignty. The 2024 sovereignty transfer treaty means that, post-transition, the Mauritius DTA network will become the competent framework for non-base islands.
The UK has ratified the OECD Multilateral Instrument (MLI), which modifies many of its existing bilateral DTAs. Whether a specific UK DTA extends to BIOT for a given transaction depends on the treaty's territorial scope clause — a qualified Tax-Adviser should review the specific situation.
Where does BIOT sit in its governance cohort?
BIOT belongs to a cohort of UK uninhabited and militarised territories with no civilian population and no domestic tax framework. Its closest governance peers are other UK Overseas Territories with restricted civilian access.
UK Overseas Territory status and the 2024 transition
BIOT is currently a UK Overseas Territory (UKoT), one of 14 such territories retained by the UK. Once the October 2024 sovereignty treaty is fully ratified, BIOT will cease to exist as a legal entity and the Chagos Archipelago will become Mauritian sovereign territory.
UK Overseas Territory. BIOT Administration (FCDO) is the competent authority. UK law applies by Order in Council. No local tax system. HMRC is the de-facto tax authority for UK nationals on the base. SOFA governs US nationals.
Mauritius sovereign territory. MRA becomes competent fiscal authority for outer islands. Diego Garcia remains under UK jurisdiction per 99-year lease. UK and US military operations continue. No change to tax position of personnel on Diego Garcia.
Currency framework
BIOT has no local currency in general circulation. The de-facto currency on Diego Garcia is the US Dollar (USD), used for base operations, commissary transactions, and contracted services.
The US Dollar is the operational currency on Diego Garcia because the base is predominantly US-administered. UK MOD procurement invoices in GBP via BFPO channels. There is no BIOT central bank and no BIOT monetary policy. Post-2024 transfer, the Mauritian Rupee (MUR) would be the formal reference for non-base islands under Mauritius sovereignty.
Common pitfalls and misconceptions
BIOT's unusual constitutional situation generates specific misconceptions for researchers, contractors, legal professionals, and anyone tracking the 2024 sovereignty transition:
The October 2024 UK–Mauritius treaty was signed but must be ratified and implemented by both governments. Until full ratification, BIOT remains a UK Overseas Territory. Filing obligations for Diego Garcia personnel remain unchanged.
The 2019 ICJ advisory opinion and ongoing litigation establish that the expulsion of Chagossians was a legally contested act. The 2024 treaty acknowledges Chagossian right of abode on outer islands. This has no bearing on current tax obligations but is relevant to any long-term civilian activity planning.
Most UK bilateral DTAs do not explicitly list BIOT as within their territorial scope. Whether a specific UK DTA applies to BIOT for a given transaction requires reading that treaty's text. A qualified Tax-Adviser should review before relying on DTA relief for BIOT-related activity.
UK Armed Forces are subject to HMRC with possible Operational Service Relief. US Armed Forces are subject to IRS with possible combat zone exclusions under IRC §112. These are separate and distinct — do not conflate them. Civilian contractors follow their own home-country framework regardless of which military they support.
Once the transition completes, Mauritius Revenue Authority (MRA) governs outer-island activity. Mauritius CIT is 15%, PIT tops at 20%, VAT is 15%. However, the outer islands will remain uninhabited initially — practical applicability of MRA rules to BIOT is prospective, not immediate.
The US–UK Status of Forces Agreement protects US personnel from UK taxation. It does not create a general tax-free zone for all contractors, civilians, or third-country nationals on the base. Each person's home-country rules apply independently of SOFA.
When-to-seek-a-pro decision flow
When should you talk to a Tax-Adviser?
Because BIOT has no domestic tax framework, the right professional is always a home-country Tax-Adviser — specifically a UK Tax-Adviser for UK Armed Forces, a US Tax-Adviser for US Armed Forces, or a home-country Tax-Adviser for civilian contractors of any nationality.
Seek qualified guidance when:
- You are UK Armed Forces or MOD civilian staff on Diego Garcia and want to confirm how Operational Service Relief applies to your deployment pay
- You are US Armed Forces on Diego Garcia and want to confirm how SOFA and IRC §112 combat zone exclusions interact with your specific pay structure
- You are a civilian contractor (non-UK, non-US) and want to confirm which country's rules apply to your Diego Garcia income
- You want to understand how the October 2024 UK–Mauritius sovereignty treaty affects any commercial or employment activity relating to BIOT
- You are researching whether a specific UK DTA extends to BIOT for a cross-border transaction
- You have received correspondence from HMRC or the IRS relating to income earned during a BIOT posting
- You are involved in the legal proceedings relating to Chagossian right of abode or compensation claims
For UK-law Tax-Advice, refer to the UK country page. For Mauritius-law Tax-Advice (post-transfer), refer to the Mauritius country page.
This page is general information. It is not personal guidance for your specific situation. Tax rules change. The 2024 sovereignty transition is ongoing — always check current status with a qualified Tax-Adviser licensed in your home jurisdiction before making any decisions.
Frequently asked
What is the BIOT tax framework?
British Indian Ocean Territory has no domestic tax framework. It is a UK Overseas Territory with no permanent civilian population. UK and US military personnel on Diego Garcia are taxed by their home-country authorities — HMRC for UK Armed Forces, IRS for US Armed Forces under the Status of Forces Agreement. No local income tax, corporate tax, or VAT exists.
What happened with the October 2024 UK-Mauritius sovereignty treaty?
On 3 October 2024, the UK and Mauritius signed a treaty transferring sovereignty of the Chagos Archipelago to Mauritius. The UK retains Diego Garcia base under a 99-year lease. The transition requires ratification by both governments and is ongoing as of 2026. Once complete, Mauritius Revenue Authority (MRA) will be the competent fiscal authority for non-base islands. Diego Garcia remains under UK and US military jurisdiction.
How are UK Armed Forces taxed when posted to Diego Garcia?
UK Armed Forces on Diego Garcia remain subject to HMRC's Self Assessment regime on the standard UK tax year (6 April to 5 April). Operational Service Relief may reduce income tax liability for qualifying deployed roles. BFPO postal routing is used for HMRC correspondence. A UK Tax-Adviser experienced in armed forces taxation can clarify applicable reliefs.
How are US Armed Forces taxed when posted to Diego Garcia?
US Armed Forces on Diego Garcia are subject to US federal income tax under IRS rules. The US-UK Status of Forces Agreement (SOFA) prevents UK from taxing US personnel. Combat zone tax exclusions under IRC §112 may apply for qualifying periods. SOFA does not create a general tax-free zone for civilian contractors or third-country nationals on the base.
What was the Chagossian forced expulsion?
When the UK created BIOT in 1965, approximately 1,500–2,000 indigenous Chagossians were forcibly relocated from the islands between 1965 and 1973 to establish the Diego Garcia base. A 2019 ICJ advisory opinion found continued UK administration was not lawful under international law. The October 2024 sovereignty treaty acknowledges Chagossian right of abode on outer islands. Ongoing displaced-persons litigation continues in UK courts.
Does the UK DTA network extend to BIOT?
The UK has approximately 130 bilateral double-tax agreements. As a UK Overseas Territory, BIOT falls within UK sovereignty, so the UK's DTA network formally applies where a treaty's territorial scope clause covers UK dependencies. Whether a specific UK DTA extends to BIOT for a given transaction requires reading that treaty's text. A qualified Tax-Adviser should review before relying on DTA relief for BIOT-related activity.
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The figures, dates, and rules on this page are sourced from the documents listed below. Where two sources disagree, both are listed.
- UK Foreign, Commonwealth and Development Office · accessed
- HM Revenue and Customs · accessed
- UK Foreign, Commonwealth and Development Office · accessed
- US Internal Revenue Service · accessed
- International Court of Justice · accessed
- OECD · accessed
Important disclaimer
Informational only — not tax advice. This page summarises publicly available information about tax in British Indian Ocean Territory as of July 2026. Tax laws change, individual circumstances vary, and the application of any rule depends on your specific facts.
TaxProsRated does not provide tax, legal, accounting, or financial advice. Before acting on anything you read here, consult a qualified tax professional licensed in your jurisdiction (in the US: CPA, Enrolled Agent, or attorney; in the UK: CIOT- or ATT-qualified adviser; in Australia: TPB-registered tax agent; elsewhere: a locally-licensed equivalent). TaxProsRated, its operators, and its contributors disclaim all liability for action taken in reliance on this page.