It is the first time Mauritius has embarked upon an expedition to the islands without seeking the permission of the United Kingdom, Prime Minister Pravind Jugnauth said in a statement.
He added it is a "concrete step" in "exercising its sovereignty and sovereign rights in relation to the Chagos Archipelago."
Those rights were strengthened in 2019 by a non-binding opinion from the International Criminal Court, which said that Britain had unlawfully carved up Mauritius, an archipelago nation whose main island is some 2000km off the southeast coast of Africa.
The Chagos islands were a part of Mauritius until Britain separated them a few years before Mauritius became independent from British colonial rule in 1968.
The United Nations General Assembly followed that opinion with a resolution two months later demanding that Britain end its "colonial administration" of the Chagos Islands, which include the US military base on Diego Garcia, and return them to Mauritius.
Even Pope Francis weighed in, saying that Britain should obey the UN resolution.
Thus far, however, Britain, which calls the archipelago a "British Indian Ocean Territory," has refused to abide by the non-binding decisions. It has argued that the Chagos archipelago has been under its sovereignty since 1814 and that its continued presence there is strategically important.
Britain's Foreign Office did not immediately respond to requests for comment on the current expedition.
In his statement, Jugnauth recalled the ICJ ruling and said that "continued administration of the Chagos archipelago by the United Kingdom constituted a wrongful act." His office did not immediately respond to an email seeking further comment.
Jugnauth has repeatedly said that ending the British administration, however, would have no implications for the US military base at Diego Garcia, which he has said Mauritius is committed to maintaining.
Britain sealed a deal in 1966 allowing the US to use Diego Garcia for defence purposes. The United States maintains a base there for aircraft and ships and has backed Britain in the legal dispute with Mauritius.
Britain evicted about 2000 people from the Chagos archipelago in the 1960s and 1970s so the US military could build its base. Many resettled in Britain and have fought in courts there to return to the islands.
Jugnauth in 2019 told the UN General Assembly their forcible eviction "remains a very dark episode of human history akin to a crime against humanity."
The vessel commissioned by Mauritius, Bleu de Nîmes, sailed on Tuesday from Seychelles to the Chagos archipelago, about 500km south of Maldives in the Indian Ocean.
Onboard for the 15-day journey is Mauritius' permanent representative to the UN, as well as legal advisers and others who planned to undertake a scientific survey at the Blenheim Reef, a partially submerged atoll in the northeastern part of the archipelago.
Jugnauth said the survey results would be part of Mauritius' submissions for a case being heard by the Hamburg-based International Tribunal for the Law of the Sea, which was brought by Maldives, which supports Britain's sovereignty claim.
Jugnauth said in his statement that he would not accompany others on the current voyage, but would personally visit the islands in a separate voyage.
