Keir Starmer has been accused of “traitorous levels of national sabotage” after it emerged he is planning to “push ahead” with the handover of the Chagos Islands to Mauritius.

Navin Ramgaloom, the Mauritian prime minister, said that the UK could end up paying his country twice as much as originally planned to seal the deal.

The islands are a British overseas territory in the middle of the Indian Ocean. Diego Garcia, the largest island, is also home to a UK-US naval base.

In October, the government announced that it was handing over sovereignty of the islands to Mauritius, but that the base would remain under British and American control on a 99-year lease.

It has been reported that the UK has agreed to pay Mauritius £90 million a year as part of the deal, which works out at £9 billion in total.

Starmer had wanted to get the handover finalised before Donald Trump - who is known to oppose the deal - was sworn in as president, but failed to do so.

Ramgaloom, who held talks with the PM last week, told the Mauritian parliament on Tuesday: “The British prime minister informed me he intends to push ahead with the agreement reached between Mauritius and the United Kingdom,” Ramgoolam said. “We remain confident it will reach a speedy resolution in the coming weeks.”

The Mauritian prime minister also said the deal has now been amended so that the UK payments are linked to the rate of inflation.

He said: “They had agreed to a package for 99 years, but not inflation-proof. The exchange rate — because it’s in dollars — would be fixed once and then last 99 years. How can that be? Any ordinary fifth-form, sixth-form student would agree that you know inflation exists.

“What is the point of having money and having half of it by the end? This is what would happen — we made the calculation.”

His comments suggest that the deal could end up costing the UK £18 billion, although Downing Street sources have rejected that calculation.

A spokesman for Starmer said: “I’m not going to give a running commentary on the deal. Once an agreement is reached, further details of the treaty will be put before both houses [of parliament] for scrutiny and treaty ratification in the usual way.”

He added: “Our position remains that finalising a deal means we can secure strong protections, including from malign influences, that will allow the base to continue to operate.

“We’ll only agree to a deal which is in the UK’s best interests and protects our national security.”

Shadow justice secretary Robert Jenrick led the Tory criticism of the Chagos deal.

Shadow foreign secretary Priti Patel said: “It seems Keir Starmer has learnt absolutely nothing - and is still putting his leftie shame of our country’s history over our national security, and our longstanding relationship with our closest ally.  

“He has 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.  

“Starmer and David Lammy must urgently explain their epic failure of diplomacy which is putting out special relationship at risk while they play pathetic gesture politics.”