Mauritius says it has submitted changes to a proposed deal over the future of the Chagos Islands, with the country's new PM saying the original agreement did not benefit his country enough.

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 that 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 the UK meanwhile, the opposition Conservative party said on Tuesday that the terms of the proposed deal were "disadvantageous to the United Kingdom".

In a letter to Sir Keir, the shadow defence and foreign secretaries said they had "grave concerns about your Government's plan to surrender sovereignty over the British Indian Ocean Territory - and to pay for the privilege of doing so".

"Our view remains that the legal justification you have used for giving away sovereignty is flawed, and it is therefore no surprise that the terms reached are so disadvantageous to the United Kingdom," James Cartlidge and Priti Patel said.

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.