(AFP) - The UN's top court will hear arguments Monday on the future status of the British-ruled Chagos Islands, home to a strategic joint US military base but a territory claimed by Mauritius.

Port Louis is set to open arguments before the International Court of Justice in a case brought by the United Nations over the Indian Ocean archipelago, which has been the centre of a dispute for more than five decades.

In a diplomatic blow to Britain, the UN General Assembly last June adopted a resolution presented by Mauritius and backed by African countries asking the Hague-based ICJ to offer a legal opinion on the island chain's fate.

The ICJ's 15 judges will now listen to arguments on the "legal consequences of (Britain's) separation of the Chagos Archipelago from Mauritius" in 1965, shortly before Port Louis' independence from its colonial ruler.

The African Union and a remarkable number of 22 countries -- which also includes the US, Germany and several Asian and Latin American nations -- are to make statements during the four-day hearing.

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 negotiations or could lay the foundation for an eventual formal claim before the ICJ -- set up in 1946 and which also rules in disputes between countries.

Mauritius, which declared independence in 1968 argues that it was illegal for London to break up its territory while still under colonial rule.

Britain detached the islands from Mauritius, then a semi-autonomous British territory, using decolonisation talks as leverage and paying £3 million pounds for them at the time.

- Key military base -

As the Cold War with the former Soviet Union intensified, London established a combined military base with the US 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.

A British diplomat in a cable at the time described it as the removal of "some few Tarzans and Man Fridays" and islanders have not been allowed to return since because of security reasons.

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, a year 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.

Mauritius on the other hand, said it wants to "eliminate colonialism" and that its independence would not be complete without getting back the Chagos Islands.

Port Louis did however say it recognised "the existence of the base and accepts its continued and future functioning in accordance with international law."

© Agence France-Presse