The British Foreign Office summoned the Iranian ambassador in protest against what London considered an Iranian violation of the sanctions imposed on Syria after reports that the Iranian oil tanker had unloaded its cargo at a Syrian port.
The Foreign Office said it was now clear that Iran had violated the assurances it had provided for the Grace 1 oil tanker, which changed its name to "Adrian Daria 1", by transporting oil to Syria, according to the British Foreign Office.
"Iran has given assurances to the Gibraltar government repeatedly that the Grace 1 will not deliver oil to anyone in Syria or anywhere else subject to European sanctions," the foreign ministry statement said.
"Iran's actions are an unacceptable violation of international rules, and Britain will submit the matter to the United Nations later this month."
British Foreign Secretary Dominique Rapp said Iran had shown total disregard for its assurances that the tanker would not unload its cargo in Syria.
The sale of Iranian oil to the brutal Assad regime is part of a behavioral pattern of the Iranian government aimed at destabilizing regional security, Rapp said in a statement.
Rapp pointed to the supply of weapons to the Houthis in Yemen and Hezbollah in Lebanon, as well as Iran's recent attempts to seize commercial ships in the Gulf.
Security forces in Gibraltar, backed by the British Royal Navy, intercepted the Iranian tanker Grace 1 off the coast of Gibraltar on 4 July.
The tanker was suspected of transporting 2.1 million barrels of oil to Syria in violation of European sanctions.
Iran has described the detention of the tanker as "piracy" and a court in Gibraltar ordered its release in mid-August despite US objections.
The US national security adviser John Bolton (who was sacked recently) announced on Friday that the tanker, which changed its name to "Adrian Draya 1" and is now flying the Iranian flag, arrived in the Syrian port of Tartus.