From Reuters:

In greetings to various delegations after a Mass on Monday in Mauritius, Francis referred to the archipelago and used the name Chagos Islands, which Britain rules as the British Indian Ocean Territory (BIOT).

Authorities saw the mention as boost for their claim and in an address to the pope on Monday night Prime Minister Pravind Jugnauth thanked him for his interest. Britain detached the Chagos Islands from Mauritius before Mauritius gained independence in 1968. London then expelled the population of the islands, then numbering about 2,000 people, to build the air base on Diego Garcia.