A UN panel has urged the UK to renegotiate a deal returning the Chagos Islands to Mauritius, saying it “fails to guarantee” the rights of the Chagossian people.

The deal, signed last month, returned sovereignty of the Indian Ocean archipelago to Mauritius, but the UK retained the right to run a military base on Diego Garcia, the largest of the Chagos Islands.

By 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 wrote.

A Foreign Office spokesperson said the UK-Mauritius deal had been “welcomed by international organisations including the UN secretary general”.

The panel of four experts were appointed by the UN Human Rights Council, but are not UN staff and are independent from the UN.

They said by the UK keeping the military base of Diego Garcia, the Chagossian people were hindered from being able to “exercise their cultural rights in accessing their ancestral lands from which they were expelled”.

The panel called for the current deal to be suspended and for a new agreement to be negotiated.

Under the agreement, the UK would pay an average of £101m a year for 99 years to continue operating the military base on Diego Garcia, in concert with the US.

The Chagos Islands are located in the Indian Ocean about 5,799 miles (9,332km) south-east of the UK, and about 1,250 miles north-east of Mauritius.

The UK purchased the islands for £3m in 1968, but Mauritius has argued it was illegally forced to give away the islands in order to gain independence from Britain.

Diego Garcia was then cleared to make way for a military base, with large groups of Chagossians forcibly moved to Mauritius and the Seychelles, or taking up an invitation to settle in England, mainly in Crawley, West Sussex.

Since then, Chagossians have not been allowed to return to Diego Garcia.

Before the UK-Mauritius deal was signed last month, two Chagossian women living in the UK – who were born on Diego Garcia – launched a last-minute legal bid to stop it, saying the agreement did not guarantee the right of return to their island of birth.

The deal includes a £40m trust fund to support Chagossians, a component that the UN panel also questioned would “comply with the right of the Chagossian people to effective remedy… and prompt reparation”.

“The agreement also lacks provisions to facilitate the Chagossian people’s access to cultural sites on Diego Garcia and protect and conserve their unique cultural heritage,” the panel added.

The Foreign Office spokesperson said: “We recognise the importance of the islands to Chagossians and have worked to ensure the agreement reflects this.”

Shadow Foreign Secretary Dame Priti Patel said the Conservatives “have been warning from the start that this deal is bad for British taxpayers and bad for the Chagossian people”.

“It is why I have introduced a bill in Parliament that would block the [agreement] and force the government to speak to the people at the heart of their surrender plans,” she said.

Both the House of Commons and House of Lords have until 3 July to pass a resolution to oppose the deal being ratified.