Mauritius told the UNs top court Monday that the British-ruled Chagos Islands were "integral" to its territory and that the Indian Ocean island chain was handed to London "under duress".
Hearings opened Monday before the International Court of Justice in The Hague where judges are hearing arguments over the future status of the remote archipelago -- home to a strategic joint British-US military base leased from Britain on territory claimed by Mauritius.
"More than 50 years after independence... the process of decolonisation of Mauritius remains incomplete," former Mauritian president Anerood Jugnauth said.
This was "as a result of the unlawful detachment of an integral part of our territory on the eve of our independence," he told the judges.
In a diplomatic blow to Britain, the UN General Assembly in June last year adopted a resolution presented by Mauritius and backed by African countries asking the ICJ to offer a legal opinion on the island chain's fate.
The Chagos archipelago was detached from Mauritius for inclusion in the newly created British Indian Ocean Territory in 1965 shortly before Port Louis' independence from Britain.
The African Union and 22 countries -- including Germany and South Africa and several Asian and Latin American nations -- will make statements during the four-day hearing.
They all voted last year whether ask the ICJ to rule on the matter. The US and Australia will also speak and are expected to support Britain while South Africa and the AU are expected to back Mauritius.
After the hearings, the ICJ will hand down a non-binding "advisory opinion", but the judges' ruling may take several months or even years.
An opinion in favour of Mauritius may strengthen Port Louis' hand in future negotiations or could lay the foundation for an eventual formal claim before the ICJ, which also rules in disputes between countries.
Britain used decolonisation talks as leverage and paid £3 million for the Chagos islands. Jugnauth said Mauritian officials were given "no room for any choice" during the 1965 independence talks.
During meetings in London, then British prime minister Harold Wilson aimed to "frighten" Mauritian officials into handing over Chagos in return for independence, Jugnauth said.
"It is against that backdrop of immense pressure and in the circumstances amounting to duress, that less than five hours later, four of the five Mauritian representatives yielded to the detachment of the Chagos Archipelago," he said.
As the Cold War with the former Soviet Union intensified, the US established a military base on Diego Garcia, the largest of the islands.
The Indian Ocean base plays a key strategic role in US military operations.
In the 1970s, it offered proximity to Asia during the fall of Saigon and the Khmer Rouge takeover in Cambodia, and as the Soviet navy extended its influence in the region.
In recent years it has served as a staging ground for US bombing campaigns in Afghanistan and Iraq.
Britain in the early 1970s also evicted the archipelago's residents -- some 2,000 in total -- to Mauritius and the Seychelles to make way for the base.
Mauritius' top lawyer in the case Philippe Sands on Monday played an emotional video of Chagossian Marie Elyse, as she tearfully described her desire to return to the "island of my birth."
"The desire to return, and the inability to do so, offer tangible evidence that the decolonisation of Mauritius is yet to be completed," Sands said.
Last year's vote before the UN whether to refer the matter to the ICJ was also seen as a test for Britain's ability to rally support from fellow Europeans at the world body, after its shock vote to leave the European Union.
The matter was passed 95-15, with 65 abstentions -- most by European member states including France, Italy and Spain.
London, ahead of Monday's hearings, pledged to mount a "robust defence" saying the move was bound to hurt relations with Port Louis.
Port Louis however says it recognised "the existence of the base and accepts its continued and future functioning in accordance with international law," a point stressed by Jugnauth on Monday.