The International Court of Justice issued an advisory opinion Monday outlining the legal consequences of separation of the Chagos Archipelago from Mauritius in 1965.

The UK and Mauritius, by virtue of the Lancaster House agreement, detached the Chagos Archipelago form Mauritius and established the British Indian Ocean Territory (BIOT). Originally, the agreement aided the US’s desire for a military base on Diego Garcia, but the UK has continued to assert control over the island. Many inhabitants were forcibly removed, and those who left voluntarily were prevented from returning.

The ICJ’s opinion, which is nonbinding, says the UK did not lawfully decolonize the islands through the Lancaster House agreement. The court further urged the UK to end its continued administration over Chagos Archipelago: “the United Kingdom has an obligation to bring to an end its administration of the Chagos Archipelago as rapidly as possible, and that all Member States must cooperate with the United Nations to complete the decolonization of Mauritius.”

The opinion follows requests from the UN General Assembly and Mauritius to return Chagos Archipelago to the country and end British control.