The deal has drawn criticism from politicians in the UK as well as the incoming US Secretary of State Marco Rubio, who said it poses a "serious threat" to US security.

Reports had suggested Mauritian Prime Minister Navin Ramgoolam would sign off the deal 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".