Iran’s detention of the vessel came two weeks after Britain seized an Iranian tanker, the Grace 1, off Gibraltar amid concerns it was breaching EU sanctions on Syria.
Tehran denounced British plans for an EU force to protect shipping in the Gulf as "provocative" and "hostile". President Hassan Rouhani said the presence of foreign forces would only increase tensions – adding that Iran and Oman were responsible for security in the Strait of Hormuz.
There’s been a muted response from the continent to British hopes of an EU naval force, with France saying it would coordinate its military assets already there, but would not send extra ones.
No end in sight
Both Britain and Iran have rejected suggestions of a tanker-swap deal to defuse the crisis, while Iran has also rejected Britain’s key demand that the 2 million barrels of oil on board the Grace 1 must not be sent to Syria.
“Iran is not subordinate to the British regime, nor to the European Union – it trades within the framework of international law,” says Seyed Mohammad Marandi, a professor at Tehran University, who was part of the Iranian delegation that helped to negotiate the 2015 nuclear deal.
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“The Syrian people have a right to fuel and if Iran seeks to carry out trade with the Syrians, that’s their business. If the British want to make ordinary Syrians suffer, that’s their business – but they do not have the right to confiscate Iranian ships or Iranian oil.”
Nuclear deal in doubt
The tanker standoff is also adding pressure to European efforts to preserve the nuclear deal, also known as the JCPOA, which was thrown into turmoil when the US pulled out last year.
Marandi says any failure of the deal would be the fault of the EU. “Iran has been abiding by the deal for the last year now, since the Americans left,” he says.
“The Europeans begged Iran to stay in the deal last year and they said they would resolve the issues within a few weeks. It’s been over a year now and the Europeans have literally done nothing.
“The Europeans should be grateful to Iran for having preserved the deal up until now.”