By Villen Anganan

PORT LOUIS (Reuters) - Mauritius Prime Minister Navin Ramgoolam has questioned the duration of the lease on a U.S.-British military base, he told a local newspaper, a contentious element of negotiations in which the UK plans to cede control of the Chagos Islands to Mauritius.

Britain struck a deal in October to hand over the Chagos Islands while retaining control of the base on Diego Garcia, the largest island of the archipelago in the Indian Ocean, under a 99-year lease. The agreement has not yet been ratified.

Having ousted the former government in a November election, Ramgoolam criticised the deal, without going into details on the individual sticking points.

Describing Britain's agreement with the former government as a "sell out", Ramgoolam told the local L'Express newspaper the lease should also be indexed to inflation and take exchange rates into account.

It should fully recognise Mauritius' ownership of the islands, he added, which could affect the UK's unilateral right to renew the lease.

"It is not only a question of money, but of our sovereignty," he said, adding that U.S. Secretary of State Antony Blinken had been keen to conclude the agreement before President-elect Donald Trump's inauguration on Jan. 20.

Marco Rubio, Trump's pick as secretary of state, has said the deal poses a threat to U.S. security by ceding the archipelago - with its base used by U.S. long-range bombers and warships - to a country he claims is allied with China.

"We are not in a hurry," Ramgoolam said. "We remain constant in our claims and will make sure that the medium and long-term interests of the country are being looked into and not only the short term."

Some Chagossians have also criticised the negotiations, saying they cannot endorse an agreement they were not involved in and have said they will protest against it.

(Reporting by Villen Anganan; Writing by Hereward Holland; editing by David Evans)