The UK government has denied claims made by the prime minister of Mauritius that it faces paying billions more under a renegotiated deal over the future of the Chagos Islands.

Last October, the UK announced it would hand over sovereignty of the islands to Mauritius but would maintain a 99-year lease over the UK-US military airbase on the largest island, Diego Garcia.

However, shortly after the deal was struck, Mauritius elected a new prime minister, Navin Ramgoolam, who wanted to reopen negotiations. He told his MPs on Tuesday new conditions had been negotiated meaning the UK’s lease payments would be linked to inflation and frontloaded.

But the UK Foreign Office said the figures being quoted were “inaccurate and misleading”.

“The UK will only sign a deal that is in our national interest,” a spokesperson said.

The Times suggested that the payments by the UK government to Mauritius could effectively double from £9bn to £18bn, but this been denied by the UK Foreign Office.

However some senior figures in government are opposed to the deal, describing it as “terrible”, “mad” and “impossible to understand”.

“At a time when there is no money, how can we spend billions of pounds to give something away?”, one senior government source said.

Progress on the deal had been paused while the UK consulted new US President Donald Trump over the proposed agreement.

US Secretary of State Marco Rubio has previously raised concerns, saying the deal posed a threat to US security, given China’s influence in the region. Mauritius has an economic relationship with China.

Environment Secretary Steve Reed said the UK government was “still waiting” for the view of the Trump administration.

He told BBC Radio 4’s Today programme that Trump’s view would “inform the negotiations”, which began under the previous Tory government.

Meanwhile, Ramgoolam said it would be “better that Trump has a look at the agreement” so the US president can see if it “is good or not”.

There had been efforts to get the deal signed before Trump’s inauguration on 20 January. However, the UK changed course, saying it was “perfectly reasonable for the US administration to consider the detail” of any agreement.

The Mauritian prime minister said he was “confident” the new deal would be approved, saying UK Prime Minister Sir Keir Starmer had said he intended to “push ahead” with the renegotiated agreement.

Speaking to Mauritian MPs on Tuesday, Ramgoolam railed against the former agreement, which he said was a “sell-out” for Mauritius.

“We have to be inflation-proof. What’s the point of getting money and then having half of it by the end? This is what would happen, we have made the calculations,” he said.

However, he did not reveal the exact amount the UK would pay, saying: “I’m not in a position to give details but let me say something, that package was very badly negotiated.”

Ramgoolam said the previous package had also been tweaked so the UK would pay more in “front-loading” at the beginning of the deal. “That also is being approved I think,” the Mauritian leader added.

Ramgoolam added that the UK would no longer be able to unilaterally act on a clause in the deal where the lease could be extended for 40 years.

This was also denied by the UK government, with the Foreign Office spokesperson saying: “There has been no change to the terms of extension in the treaty.”

Shadow foreign secretary Dame Priti Patel urged the UK government to abandon the deal, saying there had been “very little transparency”.

She told Today: “We keep hearing from the government that this is some kind of good deal – if it’s such a good deal, why are they not being honest about what the details are?

“The government of Mauritius and the people of Mauritius seem to know more about this deal than the British public, the British taxpayer and even people in our own parliament.”

Dame Priti said Sir Keir had “the audacity to tell the British people they will foot the bill and pay for the indignity of his surrender of the Chagos Islands, as he isolates the new US administration by bending the knee to Mauritius and emboldening our enemies with his disastrous surrender deal”.

Tory leader Kemi Badenoch said the prime minister should “come to Parliament and be honest with MPs” about what she called a “foolish deal”.

Reform UK leader Nigel Farage also voiced opposition to the deal, saying if the UK ceded sovereignty of the Chagos Islands, “our value to America” would become “considerably reduced”.

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 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”.

The Chagos islanders – some in Mauritius and the Seychelles, but others living in the UK – 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.