The vessel, formerly named Grace 1, was seized by British Royal Marine commandos on July 4 on suspicion of being en route to Syria.
Gibraltar released it on August 15 after receiving formal written assurances from Tehran that the ship would not discharge its 2.1 million barrels of oil in Syria.
But Britain’s foreign office said in a statement yesterday it was clear Iran had breached those assurances and that the oil had been transferred to Syria.
“Iran has shown complete disregard for its own assurances over Adrian Darya 1,” foreign minister Dominic Raab said in the statement.
“This sale of oil to (Syrian President Bashar Al Assad’s) brutal regime is part of a pattern of behaviour by the government of Iran designed to disrupt regional security.”
Britain said it had summoned the Iranian Ambassador to condemn Iran’s actions and would raise the issue at the United Nations later this month.
“Iran’s actions represent an unacceptable violation of international norms,” the statement said.
Meanwhile, the US yesterday said it was “totally unacceptable” for Iran to drag its feet in co-operating with the UN nuclear watchdog, which is seeking answers to issues that diplomats say include the discovery of uranium traces at an undeclared site.
The International Atomic Energy Agency, which is policing Iran’s nuclear deal with major powers, has called in recent days on Iran to step up its co-operation, warning “time is of the essence”.
While the watchdog has declined to comment on what prompted the warning, diplomats said inspectors had found traces of uranium at a site in Iran which Israel has described as a “secret atomic warehouse”.