An Iranian tanker that had been held by Gibraltar since July left the port last night, hours after the British territory rejected a request by the US to detain it further.
British Royal Marines had seized the ship on suspicion of it transporting oil to Syria, a close ally of Iran, in violation of European Union sanctions.
Iran has denied the claim.
The Grace 1, renamed the Adrian Darya 1, left Gibraltar at around 11pm, Refinitiv shipping data showed. Its destination was not clear.
The US made a request on Friday — a day after its detention order was lifted — for Gibraltar to continue to hold the ship.
However, the request was rejected as Gibraltar said US sanctions against Iran did not apply in the EU.
"The EU sanctions regime against Iran – which is applicable in Gibraltar - is much narrower than that applicable in the U.S.," the government said in a statement.
"The Gibraltar Central Authority is unable to seek an Order of the Supreme Court of Gibraltar to provide the restraining assistance required by the United States of America."
The initial detention of the Grace 1 started a diplomatic row that escalated when Tehran seized a British-flagged oil tanker in the Gulf two weeks later. That tanker, the Stena Impero, is still being held.
The two vessels have since become pawns in a bigger game, feeding into wider hostilities since the United States last year pulled out of an international agreement to curb Iran’s nuclear program, and reimposed economic sanctions.