The Iranian foreign ministry has summoned UK Ambassador to Iran Nicolas Hopton over what it described as the "illegal seizure" of an oil tanker carrying Iranian oil to Syria, Iranian state television has reported, citing Foreign Ministry spokesperson Abbas Mousavi.

Earlier, Spanish officials said that the Grace 1, a Panamanian-flagged supertanker carrying Iranian crude oil to Syria and seized by British patrol ships off Gibralter, was detained at Washington's behest.

Royal Marines boarded the ship, which has a 300,000 tonne carrying capacity, early Thursday morning.

Prime Minister Theresa May later praised the seizure, with a Number 10 spokesperson saying Gibralter officials' actions had sent a 'clear message' that the UK would not tolerate the violation of sanctions.

Gibraltar Chief Minister Fabian Picardo said there was evidence to suggest the tanker was attempting to deliver crude oil to the Banyas Refinery in northwest Syria, in breach of EU sanctions against Syria. The EU slapped the war-torn country with hard-hitting economic and energy sanctions in 2011.

The US, meanwhile, enacted its own tough sanctions against Iran's oil sector in 2018, and threatened to apply secondary sanctions against any country or company that continues to engage in the purchase of energy resources from the country. The UK, which remains a party to the 2015 Joint Comprehensive Plan of Action Iran nuclear deal, has not signed on to the US restrictions, although its companies have largely stopped trading in energy with the Islamic Republics in fear of economic repercussions. Last week, the European Union announced that its INSTEX financial mechanism, meant to bypass US sanctions, had become operational. Iranian officials have said however that that system "isn't enough" to resolve the economic problems faced by Tehran due to US restrictions.

Reporter’s code: 50101