PARIS — The United Nations’ highest court on Monday told Britain that it should end its control “as rapidly as possible” over a remote colonial outpost in the Indian Ocean, best known for housing a strategic American air base on the island of Diego Garcia.
The judges of the International Court of Justice in The Hague voted 13 to 1 that Britain acted unlawfully in 1965 by detaching the Chagos Archipelago from Mauritius, at the time a British colony, in order to pave the way for military installations to be leased to the United States.
The court heard testimony that negotiations between the United States and Britain about building a base began in 1964, a year before the Chagos Islands, which include Diego Garcia, were detached from Mauritius and made a new colony, named the British Indian Ocean Territory.
Officials from Mauritius, now an independent nation, said they were elated that the country’s claims were being recognized.
“We are obviously very happy with this very clear, near unanimous decision about the Chagos Archipelago,” Jagdish D. Koonjul, the ambassador of Mauritius at the United Nations, said by telephone. “We’ll be looking for further action now at the United Nations.”
Once construction started on the American air base on Diego Garcia, the largest of the handful of inhabitable islands, Britain evicted all the inhabitants not connected to the work. None of the estimated 1,800 islanders who were removed have been permitted to return.
The court’s president, Abdulqawi Ahmed Yusuf of Somalia, said a majority of judges found that Britain had violated the 1960 United Nations declaration prohibiting the breakup of colonies before independence. So the separation of the Chagos Islands from Mauritius to create a new colony was a “wrongful act,” which had not been based on a “free and genuine expression of the people concerned,” the court’s opinion said.
During the hearings, the court heard testimony that leaders of the Mauritius independence movement had signed the severance agreement in 1965 “under duress.”
In its written opinion, the court determined that Britain “has an obligation to bring an end to its administration as rapidly as possible.”
The text handed down serves only as an advisory opinion, not a binding ruling, and the court noted it had no role in deciding what steps were required to achieve an end to the British administration and to address the rights of the deported inhabitants.
The authors of the opinion included judges from a range of countries, including China and India. The single opposing vote was cast by Judge Joan E. Donoghue of the United States.
Britain’s Foreign Office, pointing out this was not a binding judgment, said it would look “carefully” looks at the details.
Because of the court’s opinion, Britain is likely to come under renewed pressure in the United Nations General Assembly, which historically has been in the forefront of decolonization issues.
The presence of the American base was not raised as a legal issue during the hearing, but experts said that any fundamental change in the status of the islands would likely require new negotiations of the lease, which was recently renewed for 20 years.
Regarding the presence of the American air base, Mr. Koonjul, the ambassador, said: “Mauritius has made it very clear that the country intends to do nothing to disrupt the American activities on Diego Garcia. We fully understand the security needs in that region.”