Britain's sovereignty over the Chagos islands and America's lease for the Diego Garcia military base could be thrown into doubt by an international court hearing due to open in Istanbul.
The case is considered of such importance that Attorney General Dominic Grieve QC, the British government's most senior law officer, will appear to defend its declaration of a marine reserve around the archipelago.
The challenge by Mauritius to the legality of the marine protected area announced by the then foreign secretary David Miliband, in April 2010, was to be heard yesterday behind closed doors by the permanent court of arbitration (PCA), a United Nations-backed tribunal that resolves disagreements between states. Its rulings are binding.
Mauritius, which launched its legal challenge three years ago, believes its success could lead to the unravelling of Britain's colonial-era claim and the eventual return of hundreds of exiled islanders who have been forced to leave the Indian Ocean archipelago. Many now live in Britain.
The PCA is based at The Hague, in the Netherlands, but its judicial proceedings are often held in neutral, international venues. Turkey is host for the latest round of the dispute. The hearing is expected to last several weeks, although Grieve will only present Britain's opening arguments.
Mauritius and the UK have hired teams of prominent British and American lawyers.
The hearing will be held in secret with none of the proceedings open to the public. At some point it is hoped the documents may be made public, including internal Foreign Office documents relating to key decisions from 1965 to April 2010.
The Mauritian prime minister, Navinchandra Ramgoolam, has claimed that the decision to establish a 141.16 million-hectare marine reserve was carried out in defiance of assurances given to him at the time by Gordon Brown, then the British prime minister, in 2009.
A Foreign Office cable released by WikiLeaks recorded the assertion by one official that "establishing a marine park would, in effect, put paid to resettlement claims of the archipelago's former residents", who were described as "Man Fridays".
In a statement to the UN's general assembly last summer, Ramgoolam said: "The dismemberment of part of our territory, the Chagos archipelago - prior to independence - by the then colonial power the United Kingdom, in clear breach of international law, leaves the process of decolonisation not only of Mauritius but of Africa incomplete."
In 1965, three years before Mauritius attained independence, Britain decided to "detach" the Chagos Islands from the rest of what was then its Indian Ocean colony. The Mauritian government, supported by every country in Africa, claims this violated UN general assembly resolution 1514, passed in 1960, which banned the break-up of colonies prior to independence.
The Chagos archipelago was subsequently declared to be part of the British Indian Ocean Territory from which, in 1971, the 1,500 islanders were deported. The largest island, Diego Garcia, was then leased to the United States as an air base. The lease is due to be renegotiated this year.
Claims that Diego Garcia was used as a secret "black site" detention centre during CIA rendition operations after 2001 resurfaced this month after Abdel-Hakim Belhaj, a rebel military commander and opponent of Muammar Gaddafi arrested in Malaysia and forcibly returned to Libya with his then pregnant wife, reported that he had been held there. The Foreign Office has disputed the claim.
The PCA case is being fought within the legal territory of the United Nations Convention on the Law of the Sea (Unclos), an area in which Britain could be at a disadvantage. While Mauritius and the Seychelles have put in mutually agreed claims for large tracts of the nearby seabed, Britain has not put in any proposals to the UN commission on the limits of the continental shelf in respect of the archipelago and has now run out of time to do so.
Britain is hoping to persuade the five arbitrators to rule that they do not have jurisdiction over the dispute.