Keir Starmer says the deal - which involves leasing the Diego Garcia base for 99 years, with the option of another 40 - is in the UK's national interest.
The Chagos Islands were separated from Mauritius in 1965, when Mauritius was still a British colony.
Britain purchased the islands for £3m, but Mauritius says it was illegally forced to give them away, as part of the deal to get independence from Britain.
In the late 1960s, Britain invited the US to build a military base on Diego Garcia, the largest island. It removed thousands of people from their homes.
The UK has come under growing pressure to return the islands to Mauritius, with both the United Nations’ top court and general assembly siding with Mauritius over sovereignty claims
Not according to them though: