A British oil tanker is sheltering off the coast of Saudi Arabia amid fears it will be seized by Iran in a tit-for-tit response to the arrest of one of its own vessels.

The British Heritage, owned by BP Shipping and registered to the Isle of Man, was due to dock in the Iraqi city of Basra on the weekend but made an unexpected U-turn.

The British-flagged ship, able to hold more than one million barrels of oil, was contracted by Royal Dutch Shell to collect the crude and deliver it to northwest Europe but the order was cancelled.

It came as Tehran warned the UK over its seizing of Iranian oil tanker Grace 1 on July 4, which it called a “threatening act which would not be tolerated.”

An Iranian Revolutionary Guards commander threatened on Friday to seize a British ship in retaliation.

A British soldiers are seen during an operation involving the oil supertanker Grace 1,Credit:
Reuters

Royal Marines impounded the tanker in Gibraltar on suspicion it was carrying oil to Syria in violation of European Union sanctions. Iran denies the vessel was headed to Banyas refinery in Syria, where the government of President Bashar al-Assad is an ally of Tehran.

Carl Bildt‏, Co-Chair European Council on Foreign Relations, questioned the legality of the UK’s move: “One refers to EU sanctions against Syria, but Iran is not a member of the EU. And the EU as a principle doesn’t impose its sanctions on others,” he tweeted. “That’s what the US does.”

Mohammad Javad Zarif, Iran’s Foreign Minister, responded with only the word “PRECISELY.”

Tensions have escalated in the Strait of Hormuz, through which a third of global seaborne oil moves, fuelling fears for the global oil market.

Maritime trackers located the British Heritage to international waters off the eastern Saudi city of Dammam on Tuesday. 

The British Heritage will not be able to exit the strait without sailing close to Iran’s coast. It is not clear what its next move is.

At a glance | Key players in Tehran

The crisis with Iran showed no signs of resolving yesterday. 

The EU on Tuesday urged Iran to reverse its scaled up uranium enrichment that breaches a nuclear arms control accord it agreed in 2015. 

Tehran has said it cannot see how European signatories – scrambling to save the deal after the US withdrew and reimposed sanctions – will manage to do so before a September deadline.

Behrouz Kamalvandi, a spokesman for Iran’s nuclear agency, hinted in a state TV interview that the Islamic Republic might consider going to 20 per cent enrichment or higher as a third step, if the material is needed and the country still has not gotten what it wants from Europe. 

That would worry nuclear nonproliferation experts because 20 per cent is a short technical step away from reaching weapons-grade levels of 90 per cent.

Experts say that since Iran’s major break from the nuclear deal there is now increased pressure on Britain and the EU to back the US by imposing heavier sanctions on Iran and by policing these sanctions more assiduously.

“Iran’s decision is likely to further antagonise the US government and can lead to further sanctions by not only the United States, but the EU and the UN,” Leigh Hansson, sanctions expert at global law firm Reed Smith, told the Telegraph.

“While the UK, France and Germany are still publicly supporting the JCPOA, the UK’s decision to seize an Iranian tanker loaded with Iranian oil and heading for Syria may indicate that Europe, or at least the UK, is more willing to object to Iran’s aggressive behaviour.”