Britain accuses Iran of transferring oil from Adrian Darya 1 vessel to Syria in breach of EU sanctions.

Britain has summoned the Iranian ambassador to the UK after accusing Tehran of breaching its assurances that the tanker held off Gibraltar in July would not transport oil to Syria in violation of European Union sanctions.

The Adrian Darya 1 vessel, formerly named Grace 1, was seized by British Royal Marine commandos on July 4 on suspicion of heading to Syria.

Gibraltar released the tanker on August 18 – despite US requests to extend the detention – after receiving formal written assurances from Tehran that the ship would not discharge its 2.1 million barrels of oil in Syria.

“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,” UK’s foreign office said in a statement on Tuesday.

“Iran’s actions represent an unacceptable violation of international norms,” it said, vowing to raise the issue at the United Nations this month.

UK’s Foreign Secretary Dominic Raab condemned the alleged sale to Syria as “part of a pattern of behaviour by Iran’s government 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.

The Adrian Darya 1 has been at the centre of a major diplomatic dispute amid heightened tensions between the United States and Iran.

The ship was blacklisted by Washington last month after being released from Gibraltar over “reliable information” that the ship was transporting oil to Syria.

After more than six weeks in detention, the vessel set sail in August from the British territory of Gibraltar despite a last-minute request by the US to stop the release.

On Sunday, Iran’s foreign ministry said the vessel had “finally docked on the Mediterranean coast and unloaded its cargo”.

An Iranian government spokesperson had earlier announced that the 2.1 million barrels of crude oil on board had been sold to an undisclosed buyer.