Oil on an Iranian tanker detained in Gibraltar has been transferred to Syria, in breach of EU sanctions, the foreign secretary has said.
The ship, seized in July with the help of UK Royal Marines, was released after Iran gave written assurances that it was not bound for Syria.
Foreign Secretary Dominic Raab said it was clear the tanker had transferred its cargo in Syria.
Mr Raab has summoned the Iranian ambassador to the Foreign Office.
The Adrian Darya One - previously called Grace One - had originally listed its destination as Turkey, when it was released after being detained in Gibraltar for six weeks.
Satellite images appeared to show the vessel off the Syrian coast on Friday.
In a statement, the Foreign Office said Iran's actions represented an "unacceptable violation of international norms" and that the UK would raise the issue at the UN General Assembly later this month.
But an Iranian foreign ministry spokesman would only say the ship had delivered its cargo after docking "on the Mediterranean coast".
The US vowed on Sunday to impose sanctions on any buyer of the 2.1 million barrels of Iranian crude oil on board.
The foreign secretary also accused Iran of illegally supplying weapons to Houthi insurgents in Yemen, supporting what he called "Hezbollah terrorists" and hijacking 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," he added.