Efforts were made to get the treaty signed before President-elect Trump’s inauguration on Monday, the BBC understands, and it had been expected the Mauritian cabinet would approve the proposal on Wednesday.

The UK plans to hand over sovereignty of the cluster of islands in the Indian Ocean, but to maintain a 99-year lease over the joint UK and US military airbase on the largest island, Diego Garcia.

A spokesman for Prime Minister Sir Keir Starmer said it was “perfectly reasonable for the US administration to consider the detail” of any agreement.

Shadow foreign secretary Priti Patel said the latest development was “complete humiliation” for the prime minister because Labour had been “desperate to sign off the surrender of the Chagos Islands before President Trump returns to office”.

The deal has drawn criticism from other politicians in the UK as well as the incoming US Secretary of State Marco Rubio, who said it poses a “serious threat” to US securityby giving the islands to a country allied with China.

Trump has not publicly commented on the deal. However Reform UK leader and Trump ally Nigel Farage told BBC Newsnight last year the agreement would damage Sir Keir’s relations with the US president-elect.

At Prime Minister’s Questions, Sir Keir defended the deal, saying the negotiations started under the last Tory government and insisting it was the best way to safeguard the military base.

Reports had suggested Mauritian Prime Minister Navin Ramgoolam would sign off an agreement on Wednesday as he attended a cabinet meeting, but it was later announced his attorney general was travelling to London to continue talks.

The UK took control of the Chagos Islands, or British Indian Ocean Territory, from its then colony, Mauritius, in 1965 and went on to evict its population of more than 1,000 people to make way for the Diego Garcia base.

Mauritius, which won independence from the UK in 1968, has maintained that the islands are its own, and the UN’s highest court has ruled, in an advisory opinion, that the UK’s administration of the territory is “unlawful”.

Conservative leader Kemi Badenoch said the prime minister was “negotiating a secret deal to surrender British territory and taxpayers in this country will pay for the humiliation”.

Badenoch said there was “no way we should be giving up British territory in Chagos”, claiming Sir Keir was “rushing a deal which will be disastrous” and it would cost British taxpayers billions of pounds.

The cost of the proposed deal to the UK has not been officially announced.

In response to Badenoch, Sir Keir told PMQs the planned agreement would ensure the military base on Diego Garcia can continue operating effectively.

A deal over the Chagos Islands was first announced in October following years of negotiations.

But weeks later, after his election, Mr Ramgoolam said he had reservations about the draft treaty and asked for an independent review.

In a joint statement in October, Mauritius and the UK said the deal would “address wrongs of the past and demonstrate the commitment of both parties to support the welfare of Chagossians”.

The Chagos islanders – some in Mauritius and the Seychelles, but others living in Crawley in Sussex – do not speak with one voice on the fate of their homeland.

Some have criticised the deal, saying they were not consulted in the negotiations.

Under the proposed deal, Mauritius will be able to begin a programme of resettlement on the Chagos Islands, but not on Diego Garcia.

UK Foreign Secretary David Lammy has previously played down the criticism, saying it is a “very good deal” for “our national security” because it secured the legal basis of the Diego Garcia military base.

Under the terms of the original agreement, which was announced in October, the UK would relinquish sovereignty to Mauritius over the archipelago but maintain a 99-year lease over Diego Garcia, home to a major UK-US military airbase.It was finalised shortly before an election which then-Prime Minister Pravind Jugnauth lost in a landslide defeat.

His successor Navin Ramgoolam has since said he had reservations about the deal, which has also attracted criticism from the opposition Conservative party in the UK and officials in the incoming Trump administration.

When the agreement was first made public after years of talks, UK Prime Minister Sir Keir Starmer and Jugnauth called it a “seminal moment in our relationship and a demonstration of our enduring commitment to the peaceful resolution of disputes and the rule of law”.

The British government has said that it also “protects the long term secure operation of [the military] base”.

Soon after coming into office last month, however, Ramgoolam asked legal experts to review the terms of the deal.

Speaking in the Mauritian parliament on Tuesday, Ramgoolam said that his government had submitted counter-proposals aimed at ensuring that the final agreement aligned with the nation’s interests.

“During the discussions, Mauritius made clear that while it is still willing to conclude an agreement with the United Kingdom, the draft agreement which was shown to us after the general elections is one which, in our view, would not produce the benefits that the nation could expect from such an agreement,” Ramgoolam told MPs.

He did not spell what the counter-proposals were or the exact nature of the objections.

Ramgoolam confirmed that the UK’s response to these proposals was received on Monday and is currently under review.

A delegation of senior officials from the UK, led by Harriet Mathews, director general for Africa, the Americas, and Overseas Territories at the Foreign Office, visited Mauritius last week.

Accompanied by British High Commissioner Charlotte Pierre, she met Ramgoolam on 11 December to further the negotiations.

“We remain confident the agreement is in both sides interests and will continue to work with the new Mauritius government to finalise the deal. We’ve always said we’ll engage with the new administration in order to finalise the deal,” Sir Keir’s spokesperson said, adding that there was no timeline on the deal.

The only hint at what might be the issue from the Mauritian side are comments made last month by Arvin Boolell, the minister of agro-industry and fisheries.

He criticised the former prime minister for granting the UK a long lease over Diego Garcia – he said it was 200 years, though the publicised timeframe was an initial period of 99 years.

“In other words,” Boolell remarked to a newspaper, “the tenant has become the owner of Diego Garcia for 200 years.

“In recent years, the UK has faced rising diplomatic isolation over its claim to what it refers to as the British Indian Ocean Territory, with various United Nations bodies – including its top court and general assembly – overwhelmingly siding with Mauritius and demanding the UK surrender what some have called its “last colony in Africa”.

The government of Mauritius has long argued that it was illegally forced to give the Chagos Islands away in return for its own independence from the UK in 1968.

At the time, the British government had already negotiated a secret deal with the US, agreeing to lease it the largest atoll, Diego Garcia, for use as a military base.

Britain later apologised for forcibly removing more than 1,000 islanders from the entire archipelago and promised to hand the islands to Mauritius when they were no longer needed for strategic purposes.Until very recently, the UK insisted that Mauritius itself had no legitimate claim to the islands.

Under the agreement, which has still to be signed, the UK would relinquish sovereignty over the archipelago but maintain a 99-year lease over Diego Garcia, home to a major UK-US military airbase.

A spokesperson for UK Prime Minister Keir Starmer said on Thursday that Navin Ramgoolam had “indicated he was open” to the historic deal.

Ramgoolam, who was elected prime minister of Mauritius two weeks ago, told the Mauritian parliament he had asked for a review and that the findings will be considered by the cabinet.

When the deal was signed, after years of talks, Sir Keir and his then-Mauritian counterpart Pravind Jugnauth called it a “seminal moment in our relationship and a demonstration of our enduring commitment to the peaceful resolution of disputes and the rule of law”.

But in his election campaign, Ramgoolam and his allies in the Change coalition described the agreement as a “sell-out” motivated by desperation ahead of the vote.

Arvin Boolell, the newly appointed minister of agro-industry and fisheries, had been more specific about the objections in his comments on Monday.

He criticised the former prime minister for granting the UK a long lease over Diego Garcia – he said it was 200 years, though the publicised timeframe was an initial period of 99 years.

The Change coalition are not the only ones with reservations about the deal – Marco Rubio, US President-elect Donald Trump’s pick for secretary of state, said in October that the deal posed a “serious threat” to US national security by giving the islands to a country allied with China.

Some groups representing the interests of Chagossians have also expressed disquiet, saying they were excluded from the negotiations.

The British prime minister and his Foreign Secretary, David Lammy, have defended the agreement as a “good deal”.

“I’m very, very confident that this is a deal that the Mauritians will see, in a cross-party sense, as a good deal for them,” Lammy said on Thursday.

The deal is still subject to the finalisation of a treaty.Details of the treaty’s legal text are being worked out and it is expected to go before the UK Parliament for scrutiny next year.