Britain's deal to hand the Chagos Islands to Mauritius fails to guarantee the rights of the Chagossian people, a group of United Nations experts said on Tuesday.

The agreement announced last month maintains British and US control of a key military base on the Indian Ocean archipelago's largest island, Diego Garcia.

The deal will see Britain pay Mauritius £101 million ($137 million) annually for 99 years to lease the facility.

As part of the agreement, Mauritius will be able to resettle Chagossian islanders, expelled from the archipelago by Britain in the 1960s, to all of its more than 50 islands, apart from Diego Garcia.

Right to return

"By maintaining a foreign military presence of the United Kingdom and the United States on Diego Garcia and preventing the Chagossian people from returning to Diego Garcia, the agreement appears to be at variance with the Chagossians' right to return," the UN experts said in a joint statement.

It also "hinders their ability to exercise their cultural rights in accessing their ancestral lands from which they were expelled," they added.

The statement was penned by the UN special rapporteurs on minority issues, on contemporary racism, and on reparation, plus the chair of the working group of experts on people of African descent.

UN experts are appointed by the Human Rights Council to report their findings. They are independent figures and therefore do not speak for the United Nations itself.

Trust fund

Under the deal, Britain will set up a £40-million trust fund for the 10,000-strong Chagossian diaspora.

The UN experts raised concerns about whether the fund would comply with the right of Chagossians to effective remedy and adequate, effective, and prompt reparation.

The deal does not provide restitution, satisfaction and guarantees of non-repetition, the experts claimed.

"In light of these significant concerns, we call for the ratification of the agreement to be suspended and for a new agreement to be negotiated that fully guarantees the rights of the Chagossian people to return to all islands of the Chagos archipelago, including Diego Garcia," they said.

'Gravely concerned'

The experts said they had raised their concerns with Britain and Mauritius.

"We are gravely concerned about the lack of meaningful participation of Chagossians in processes that have led to the agreement," they added.

Last month, Human Rights Watch said the deal could "entrench" the exile of Chagossians from Diego Garcia.