Britain has accused Tehran of an "unacceptable" breach of international norms after it apparently broke a promise that an oil tanker detained off Gibraltar this summer would not deliver oil to Syria.
Dominic Raab, the Foreign Secretary, summoned the Iranian ambassador on Tuesday afternoon following reports that the Adrian Darya 1, which was at the centre of a diplomatic crisis after being seized by Royal Marines in July, had delivered a cargo of crude oil to the Syrian port of Tartus.
Britain says Iran repeatedly gave assurances that the ship would not deliver oil to any EU-sanctioned entity in Syria or elsewhere before it was released last month.
Mr Raab said: “Iran has shown complete disregard for its own assurances over Adrian Darya 1.
“This sale of oil to Assad’s brutal regime is part of a pattern of behaviour by the Government of Iran designed to disrupt regional security. This includes illegally supplying weapons to Houthi insurgents in Yemen, support for Hezbollah terrorists and most recently its attempts to hijack commercial ships passing through the Gulf.”
“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.”
The Adrian Darya 1, known as the Grace 1 until it was renamed by its owners last month, was seized by Gibraltar authorities and Royal Marine Commandos acting on intelligence that it was bound for Syria on July 4.
Britain and Gibraltar said the move was to enforce European Union sanctions that forbid the supply of oil to the regime of Bashar al-Assad, the Syrian dictator.
It was released in August after a court in Gibraltar accepted assurances that the vessel would not breach the sanctions, and rejected a last-minute US bid to have it impounded.
But the vessel spent several days meandering near the Syrian coast and turned off its transponder before apparently making its delivery last week.
John Bolton, the US national security adviser, on Friday alleged that the tanker had arrived at the Syrian port of Tartus.
Satellite photographs obtained by the AP on Tuesday showed the vessel remained moored near the port.
The Foreign Office said the delivery was an "unacceptable violation of international norms" and that Britain would raise the issue at the United Nations later this month.
The seizure of the Adrian Darya sparked a diplomatic crisis that raised fears of a return to the 1980s "tanker wars" in the Persian Gulf.
Iran condemned the move as an act of "piracy" and accused Britain of acting at the behest of the United States, which is trying to cripple Iranian oil exports as part of a policy of economic "maximum pressure" against Tehran.
In what appeared to be a tit-for-tat move, Iranian commandos seized the Stena Impero, a British flagged tanker sailing in the Persian Gulf on the pretext of violating international maritime rules in mid-July.
Iran continues to hold the British vessel. The Iranian foreign ministry said on the weekend that the ship could soon be released as legal proceedings are concluded.
The Royal Navy has deployed three warships to the region to protect British shipping.