The legality of Britain's claim to the Chagos Islands in the Indian Ocean, which house a major U.S. military base, will be considered by the International Court of Justice on Monday when it starts a week of hearings.
Britain has overseen the islands since 1814 and in 1965 detached the Chagos Islands from Mauritius, a British colony that became independent three years later.
Mauritius has said it was unfairly pressured to cede control of the islands during the negotiations, and had agreed only to their temporary use for military purposes.
Mauritius petitioned the United Nations for an International Court of Justice opinion on the legality of Britain's possession of Chagos.
Britain and the United States opposed the demand, but were voted down by the U.N. general assembly. Britain is expected to argue that Mauritius is trying to improperly use the International Court of Justice or "World Court" to settle a bilateral dispute.
The World Court will ultimately issue only an "advisory opinion" on the case, but opinions by the United Nations' highest court can carry significant weight in international law.