Royal Marines have boarded an oil tanker on its way to Syria thought to be breaching EU sanctions, the government of Gibraltar has said.
Authorities said there was reason to believe the ship - Grace 1 - was carrying Iranian crude oil to the Baniyas Refinery in Syria.
The refinery is subject to European Union sanctions against Syria.
Britain's ambassador in Tehran, Robert Macaire, has been summoned over the incident.
Iran's foreign ministry spokesman Abbas Mousavi was quoted on Iranian state TV as saying the ambassador had been summoned over the "illegal seizure" of the tanker.
Gibraltar's chief minister, Fabian Picardo, praised the marines who detained the ship.
"Be assured that Gibraltar remains safe, secure and committed to the international, rules-based, legal order," he said, thanking the police, customs and port authorities for their involvement in detaining the ship.
Gibraltar port and law enforcement agencies detained the super tanker and its cargo on Thursday morning with the help of the marines.
The BBC has been told a team of about 30 marines, from 42 Commando, were flown from the UK to Gibraltar to help seize the tanker, at the request of the Gibraltar government.
A defence source described it as a "relatively benign operation" without major incident.
Mr Picardo said he had written to the presidents of the European Commission and European Council to give details of the sanctions that have been enforced.
The Baniyas refinery, in the Syrian Mediterranean port town of Tartous, is a subsidiary of the General Corporation for Refining and Distribution of Petroleum Products, a section of the Syrian ministry of petroleum.
The refinery has been subject to EU sanctions since 2014.
A spokesman for Prime Minister Theresa May said she welcomed the "firm action" by the Gibraltarian authorities.