The text of the treaty says that Britain “agrees to expeditiously inform Mauritius of any armed attack on a third state directly emanating from the base on Diego Garcia”.

Whitehall officials said any notification would be given after an attack took place, not before. However, the treaty does not explicitly say whether the notification should come before a strike.

Philippe Sands, an international lawyer who previously acted for Mauritius against the UK, told the House of Lords last week that there were “presumably different interpretations” of the text, and that it could be interpreted by Mauritius that Britain should give notice before an attack took place.

The clause has raised alarms among critics of the Chagos deal, which has not yet come into force. The terms of the agreement will not affect any action against Iran by the US in the next few days, which Mr Trump is considering.

Asked on Thursday whether the UK would block America from using Diego Garcia for an attack, a Downing Street spokesman said: “I’m not going to get into hypothetical situations and I’m not going to speculate.”

Mr Trump has indicated he would be open to attacking Iran directly, but has yet to make a final decision.