(CNN)The UK must return the Chagos Islands to Mauritius "as rapidly as possible," the United Nations' highest court ruled Monday, branding its occupation of the Indian Ocean archipelago illegal.
The islands, which are home to US military base Diego Garcia, were separated from the former British territory of Mauritius during decolonization in 1968. The international Court of Justice (ICJ) has ruled that act was illegal under international law.
For years, the US base has been vital to the military, serving as a landing spot for bombers that fly missions across Asia, including over the South China Sea. The UN ruling raises questions about its future.
The decision by the ICJ is merely advisory. The matter of who holds sovereignty over the Islands, located more than 2,000 miles off the east coast of Africa, will now be debated by the United Nations General Assembly -- which referred the case to the ICJ despite London's protests.
"The United Kingdom is under an obligation to bring an end to its administration of the Chagos Archipelago as rapidly as possible, thereby enabling Mauritius to complete the decolonization of its territory in a manner consistent with the right of peoples to self-determination," the ICJ said in its judgment.
Carl Schuster, a former director of operations at the US Pacific Command's Joint Intelligence Center, said the ruling could force Washington to negotiate with Mauritius over the future of the Diego Garcia base.
"Everything boils down to what Britain does," he said. "If it transfers the islands to Mauritius -- and it has a history of obeying these rulings -- then it's up to Mauritius. If they say the existing agreement is no longer valid, then (the US) would have to renegotiate."
Schuster said the Indian Ocean base was "very important to US operations in the Persian Gulf and Indian Ocean" and its loss could have a major impact, forcing the US "to change logistics support" in the region.
"It wouldn't weaken (US military strength) necessarily but logistics are everything," he added.
According to the US Navy, Diego Garcia was used to guide tactical aircraft supporting US military missions in Afghanistan and Iraq, and featured remote satellite tracking stations, an Air Force Space Command and Pacific Air Force support and logistics teams.
File photo of Diego Garcia, largest island in the Chagos archipelago and site of a major United States military base in the middle of the Indian Ocean leased from Britain in 1966.
In a statement following the IC, Mauritian Prime Minister Pravind Jugnauth urged the UK to "respect" the court's "clear, precise and very strong opinion."
A representative for the UK's Foreign Office said in a statement that "this is an advisory opinion, not a judgment."
"The defense facilities on the British Indian Ocean Territory help to protect people here in Britain and around the world from terrorist threats, organized crime and piracy," the representative said.
Illegal separation
An island nation in the Indian Ocean, Mauritius was a British colony from 1810 until 1968, when it achieved independence from London and became a republic.
Several years before independence, the UK began talks with the US on the "strategic use of certain small, British-owned islands in the Indian Ocean" for defense purposes, the court said.
"During these talks, the United States expressed an interest in establishing a military communication facility on Diego Garcia," the court added.
London decided that during the decolonization process, the Chagos archipelago would be separated from the rest of Mauritius and incorporated into a separate colony, the British Indian Ocean Territory.
"The detachment (or excision) of the Chagos Archipelago was carried out without any regard to the will of the people of Mauritius, including those who lived in the Chagos Archipelago," Mauritius said in a submission to the court.
"The administering power had already decided that the territory would be excised and turned into a new colony, in order to allow one of its allies to build a military base on the island of Diego Garcia."
While representatives of Mauritius agreed to the separation at the time, the current government claim they did so under "duress."
As part of the separation process, the entire population of the Chagos Islands was expelled and prevented from returning, a situation the UK has since said "was shameful and wrong."
In the 1980s, the UK paid an estimated $5.2 million to more than 1,300 evicted islanders, on the condition they sign or place a thumbprint on a form renouncing their right to return to the Chagos archipelago.
Today, many of those Chagossians are dispersed in several countries, including the United Kingdom, Mauritius and Seychelles, unable to return, according to the ICJ.
'Searing indictment' of the UK
The matter will now return to the UN General Assembly for debate, where the UK is unlikely to prevail. In 2017, the assembly passed a resolution to refer the matter to the ICJ by a vote of 94 to 15, with 65 abstentions.
Multiple European and other allies of the UK did not support it in that vote, with Germany, France, Canada abstaining, while most former colonial nations voted in favor of sending the case to the ICJ.
"The full decolonization of Mauritius, and of Africa, is long overdue," Namira Negm, who represented the African Union in the court case, said in a statement. "The ICJ has made it clear that this must be accomplished today and not tomorrow. Only then the Africans can be free and the continent can aspire to live free of colonialism."
Jeremy Corbyn, leader of the UK opposition Labour party, said that "over 40 years ago the Chagos Islanders were disgracefully forced from their homes by the UK government -- to make way for a US military base."
"It's fantastic to see the (ICJ) take crucial steps to correct this injustice and uphold the right of the Chagossians to return home," he said.