Britain accused Iran on Tuesday (September 10th) of breaching its assurances that a tanker held off Gibraltar this summer would not transport oil to Syria, and summoned the Iranian ambassador to protest, AFP reported.
"It is now clear that Iran has breached these assurances and that the oil has been transferred to Syria and (President Bashar al-Assad's) murderous regime," the Foreign Office said.
It said Britain would raise the issue at the UN, with Foreign Secretary Dominic Raab warning that it was part of a "pattern of behaviour... designed to disrupt regional security".
"We want Iran to come in from the cold but the only way to do that is to keep its word and comply with the rules-based international system," he said.
Gibraltar security forces aided by British Royal Marines intercepted the Grace 1 Iranian supertanker off the coast of the British overseas territory on July 4th.
It was suspected of shipping its 2.1 million barrels of oil to Syria in breach of EU sanctions.
Iran reacted with fury to the seizure, saying it was an act of "piracy". A Gibraltar court ordered its release in mid-August against US objections.
US national security adviser John Bolton on Friday alleged that the tanker, which had set sail after its release flying the Iranian flag and renamed the Adrian Darya, had arrived at the Syrian port of Tartous.
"Iran repeatedly gave assurances to the government of Gibraltar that the Grace 1/Adrian Darya 1 would not deliver oil to any EU-sanctioned entity in Syria or elsewhere," the Foreign Office statement said.
"Iran's actions represent an unacceptable violation of international norms and the UK will raise the issue at the UN later this month."