A detachment of Royal Marines in Gibraltar has detained an oil tanker suspected of carrying oil to Syria in breach of European Union sanctions.

The government in the British overseas territory said port and law enforcement agencies, assisted by the Royal Marines, boarded the Grace 1 supertanker on Thursday.

“We have detained the vessel and its cargo. This action arose from information giving the Gibraltar government reasonable grounds to believe that the vessel, the Grace 1, was acting in breach of EU sanctions against Syria,” said Fabian Picardo, Gibraltar’s chief minister.

“We have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas refinery in Syria.”

He added: “With my consent, our port and law enforcement agencies sought the assistance of the Royal Marines in carrying out this operation.”

Syrian government offensive in Idlib Show all 8 1 /8 Syrian government offensive in Idlib Syrian government offensive in Idlib Syrians, including a member of the White Helmets civil defence rescue teams, run for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians search for survivors amid the rubble in a freshly bombarded area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians search for survivors amid the rubble in a freshly bombarded area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians look for survivors in a heavily damaged area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib This photo provided by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting, shows Civil Defense workers searching for victims under the rubble of a destroyed building after an airstrike by Syrian government forces, Maaret al-Numan, a town in south Idlib, Syria, Thursday, May 30, 2019. Rescue workers and activists say at least five, including three from the same family, have been killed in Syria's last rebel stronghold when warplanes targeted the building they live in, leveling it. AP Syrian government offensive in Idlib A Syrian girl cries as she runs for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib Syrians run for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib A man rescues a child while members of the Syrian civil defence known as the White Helmets search the area for survivors following following a reported air strike by regime forces and their allies on Maaret al-Noman in Syria's southwestern Idlib province. AFP/Getty

The EU imposed sanctions on the country in 2011, shortly after Bashar al-Assad‘s brutal crackdown on pro-democracy demonstrators spiralled into civil war.

Prime minister Theresa May’s official spokesperson said: “We welcome this firm action to enforce EU sanctions against the Syrian regime and commend the Gibraltarian authorities involved in successfully carrying out this morning’s operation.

“This sends a clear message that violation of the sanctions is unacceptable.”

The Foreign Office also welcomed the “firm action” by the Gibraltarian authorities.

Gibraltar detained the supertanker after a request made by the United States to Britain, Josep Borrell, Spain’s acting foreign minister, said.

Spain was looking into the seizure of the ship and how it may affect Spanish sovereignty as it appeared to have been seized in Spanish waters, Mr Borrell said.

Spain does not recognise the waters around Gibraltar as British.

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Mapping data from Refinitiv Eikon indicates the Grace 1 sailed from Iran, taking a longer route around the southern tip of Africa rather than going through Egypt’s Suez Canal.

The 300,000-tonne tanker was documented as loading fuel oil in Iraq in December, though the Iraqi port did not list it as being in port and its tracking system was switched off.

The tanker then reappeared on tracking maps near Iran’s port of Bandar Assaluyeh, fully loaded.

If its cargo was confirmed to be Iranian crude oil, its attempted delivery to Syria could also violate US sanctions on Iranian oil experts.