The case is seen by some as a test of whether colonial-era deals struck by world powers are legitimate.

Mauritius has told the United Nations’ top court that former colonial power the UK unduly pressured it in 1965 to give up a remote Indian Ocean island chain in exchange for independence.

Judges at the International Court of Justice (ICJ) on Monday began hearing arguments for an advisory opinion the UN General Assembly requested on the legality of British sovereignty over the Chagos Islands.

The largest island, Diego Garcia, has housed a major US airbase for decades.

“More than 50 years after independence … the process of decolonisation of Mauritius remains incomplete,” Anerood Jugnauth, former Mauritian president, told the judges at The Hague.

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.

Judges at the ICJ, or “World Court”, are expected to spend three more days hearing representatives from 22 countries and the African Union arguing over colonial history and the rights of exiled islanders to return.

The majority of these states oppose Britain’s assertion that it has sovereignty over the island, but the United States, Australia and Israel are expected to support the UK.

No date has been set for a decision. Although the ICJ’s opinions are not binding, they carry great weight under international law.

‘We are still suffering’

The case is seen by some as a test of whether colonial-era deals struck by great powers and weaker states are legitimate, given the power imbalance.

Britain in 1965 detached the Chagos Islands from Mauritius, a British colony that became independent three years later.

It leased Diego Garcia to the US in 1966, clearing the way for the construction of the airbase that required the forced removal of around 1,500 people. The islanders have never been allowed to return home.

Mauritius’ government claims this was in breach of UN resolution 1514, which banned in 1960 the breakup of colonies before independence.

Today, Diego Garcia is believed to be hosting one of the largest US military bases in the world, and home to an estimated 4,000 troops.

The UK, which has yet to respond, is expected to argue that Mauritius is trying to improperly use the ICJ to settle a bilateral dispute.

Its position is that it has the right to use the islands as long as they are needed for military purposes and refuses to give a date for when it plans to return the islands.

The people displaced from the Chagos Islands have lobbied for years for their return. But in 2016, the UK’s foreign ministry extended Diego Garcia’s lease until 2036, and declared the expelled islanders would not be allowed to go back.

Outside the court, a small group of Chagossians gathered to protest.

They unfurled banners denouncing “modern slavery” and called for Chagossians to be allowed self-determination.

“I want the world to see that we are still suffering,” Isabelle Charlot, whose father was born on Chagos, told the Reuters news agency.

Speaking to Al Jazeera from London, Tom Guha, chairman of the UK Chagos Support Association, which has campaigned for years for the islanders’ right of return, said Britain and the US “need to take responsibility for their actions over the last 50 years, even if sovereignty is given to Mauritius”.

“We believe the question of sovereignty has no bearing whatsoever on the question of return. We know that the right of return is practically feasible and whichever way the decision goes, the right of return must be granted.”