Britain said Iran had sold the oil cargo of tanker Adrian Darya 1 to Syria, breaking assurances it had given over the vessel which had been detained in Gibraltar for a suspected breach of European Union sanctions.
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 Aug. 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 on Tuesday 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.
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