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Foreign Secretary Jeremy Hunt outlined the plans to parliament after a meeting of COBR, the government's emergency committee, which discussed London's response to Friday's capture of the Stena Impero tanker by Iranian commandos at sea.
"Under international law, Iran had no right to obstruct the ship's passage - let alone board her. It was therefore an act of state piracy," Hunt said.
"We will now seek to put together a European-led maritime protection mission to support safe passage of both crew and cargo in this vital region."
The British announcement signals a potential shift from Washington's major European allies who so far have been cool to US requests that they beef up their military presence in the Gulf, for fear of feeding the confrontation there.
It is unclear how much influence Britain may have in Europe given it is about to have a new Prime Minister - widely expected to be Boris Johnson - who takes over a country divided over Brexit, its planned departure from the European Union.
Hunt made a point of saying that the maritime protection proposal would not involve contributing European military power to back Washington's hardline stance against Iran.
The proposed new maritime protection mission "will not be part of the US maximum pressure policy on Iran because we remain committed to preserving the Iran nuclear agreement", he said.
WARNING FOR WEST
Iran's foreign minister warned the West on Monday against "starting a conflict," saying it was not seeking confrontation.
Speaking in Nicaragua, Foreign Minister Mohammad Zarif said Iran had taken measures against the ship to implement international law, not in retaliation for the British capture of an Iranian tanker two weeks earlier in Gibraltar.
"Starting a conflict is easy, ending it would be impossible," Zarif told reporters after meeting his Nicaraguan counterpart.
"It's important for everybody to realise, it's important for Boris Johnson to understand, that Iran does not seek confrontation," he said. "Iran wants to have normal relations based on mutual respect."
Zarif said Iran acted when it observed that the UK ship did not follow regulations.
"The UK ship had turned down its signal for more time than it was allowed to (and) was passing through the wrong channel, endangering the safety and security of shipping and navigation in the Strait of Hormuz, for which we are responsible."
In contrast, he described the seizure of the Iranian ship as "piracy" and "violation of international law" by British and Gibraltar authorities.
US TARGETS CHINESE COMPANY
Washington's major European allies Britain, France and Germany all opposed US President Donald Trump decision last year to abandon an international agreement that gave Iran access to trade in return for accepting curbs on its nuclear programme.
In the latest element of the US pressure campaign, the United States said it had sanctioned China's state-run energy company Zhuhai Zhenrong Co Ltd for allegedly violating restrictions imposed on Iran's oil sector, US Secretary of State Mike Pompeo said.
The Europeans have tried to stay neutral as tensions have risen between Tehran and Washington. But Britain was plunged directly into the crisis on July 4 when it seized an Iranian tanker off Gibraltar accused of violating sanctions on Syria.
Iran repeatedly threatened to retaliate, culminating with its seizure of the Stena Impero on Friday using the same tactics - commandos rappelling to the deck from helicopters - that British Royal Marines had used aboard Iran's own ship.
The US has an aircraft carrier strike group in the Gulf and a navy that is far more powerful than those of all its European allies combined. But it has made a point in recent weeks of saying it cannot protect shipping on its own.
Asked about the capture of the British ship, Pompeo said: "The responsibility ... falls to the United Kingdom to take care of their ships."
Almost a fifth of the oil consumed globally passes through the Strait of Hormuz.
IRAN SAYS IT HAS ARRESTED SPIES
In a separate development that could generate more tensions, Iran said it had arrested 17 spies working for the CIA and sentenced some of them to death, an announcement Trump called "totally false".
Iranian state television published images it said showed CIA officers who had been in touch with the suspected spies. The Ministry of Intelligence said the 17 spies had been arrested in the 12 months to March 2019.
Such announcements are not unusual in Iran and are often made for domestic consumption, although the timing suggests the possibility of an intention to link it to the Gulf crisis.
Trump later told reporters it was getting harder for him to want to make a deal with Iran.
Trump has said the 2015 nuclear deal reached by his predecessor Barack Obama is too weak and new sanctions are needed to force Iran to renegotiate it. Tehran has said it cannot negotiate unless Washington lifts sanctions and abides by the agreement.
Tensions have risen since May, when the United States closed sanctions loopholes to try to halt all Iranian oil exports. Since then Washington has accused Tehran of attacking ships and Iran has breached limits in the deal.