The tanker was impounded in Gibraltar on the southern tip of Spain after sailing around Africa, according to Reuters.
Though the ship flies a Panama flag and is listed as managed by a Singaporean company, Iran’s Foreign Ministry summoned the British ambassador to declare "its very strong objection to the illegal and unacceptable seizure."
The Gibraltar government said it had good reason to believe the tanker was carrying crude oil to the Baniyas refinery in Syria.
"That refinery is the property of an entity that is subject to European Union sanctions against Syria," Gibraltar Chief Minister Fabian Picardo told Reuters. "With my consent, our port and law enforcement agencies sought the assistance of the Royal Marines in carrying out this operation."
Europe has banned oil shipments to Syria since the start of its civil war in 2011 but has never before seized a tanker.
"This is the first time that the EU has done something so public and so aggressive. I imagine it was also coordinated in some manner with the U.S. given that NATO member forces have been involved," Matthew Oresman, a partner with law firm Pillsbury Winthrop Shaw Pittman who advises firms on sanctions, told Reuters.
Related: Reuters - U.S.'s Bolton welcomes Britain's seizure of tanker with Iranian oil