Next week the issue will come before judges at the International Court of Justice (ICJ) in The Hague.
“We have had verbal threats,” said the Prime Minister of Mauritius, Pravind Jugnauth, in an interview with BBC News.
He did not dispute a report that Britain’s former Foreign Secretary Boris Johnson had called him personally to pressure Mauritius to back down on its demand that the islands be returned after decades under UK control.
“Unfortunately, we have been threatened with retaliation… on issues of trade and on issues of investment, you know, and on our relationship with the UK,” Mr Jugnauth added.
It is half a century since the UK took control of the Chagos Islands from its then colony, Mauritius, and evicted the entire population, before inviting the United States to build a military base on Diego Garcia, one of the larger atolls.
Mauritius was in the middle of negotiating its independence from the UK at the time and has repeatedly condemned the deal.
Although the UK says it will hand the islands back to Mauritius when they are no longer required for defence purposes, the Mauritian government appears to be losing patience.
It recently embarked on a diplomatic campaign to win international support for the islands’ return.