The foreign secretary, Jeremy Hunt, is under intense pressure to join US-led plans for an international maritime protection force in the Gulf as signs grow that Iran is preparing for a long standoff over the British-flagged tanker it has detained.
As Tehran signalled it would refuse to release the Steno Impero until the UK released an Iranian-flagged ship seized off the coast of Gibraltar a fortnight ago, the British government faced accusations it had failed to sufficiently guard its shipping in the Gulf.
Adding to the tensions, the defence minister, Tobias Ellwood, said cuts had left the Royal Navy too small to manage Britain’s interests around the globe.
Senior Tory backbenchers including figures close to Boris Johnson, who is expected to become prime minister this week, are angry at the government’s reluctance to accept an offer from Washington to protect British vessels in the region. The capture of the Stena Impero on Friday was widely seen as an avoidable error for which ministers had ample warning.
Iain Duncan Smith, tipped for a cabinet post in a Johnson administration, said the government needed urgently to answer the charge that it had made an error. He said genuine questions had been raised about the UK’s contingency planning after the UK seized the Iranian tanker, Grace 1, on 4 July. “If something didn’t send an alarm signal that we needed to have serious assets or protection and convoying of our vessels in that area then I want to know why not.”
Duncan Smith said his sources had suggested Washington had invited the UK “to use US assets to support British shipping and they were not taken up at that point.” He said he wanted to know why additional UK assets were not speedily dispatched.
Hunt is expected to announce limited sanctions directed against members of the Iranian Revolutionary Guards and to say that the UK is willing to join an international protection force on the right terms. It will also send air assets to the region.
The Foreign Office is seeking a diplomatic solution to the conflict but has advised all British-flagged ships not to enter the Strait of Hormuz.
Bob Sanguinetti, the chief executive of the UK Chamber of Shipping, said: “The health of global trade relies on freedom of navigation. We must now arrive at a diplomatic solution with the aim of de-escalation. We must restore confidence in the security of ships passing through the strait, and if this necessitates the deployment of more naval vessels from the international community, we would welcome that.”
Theresa May will chair a meeting of COBR on Monday morning. As well as receiving the latest updates from ministers and officials, the COBR meeting will discuss the maintenance of the security of shipping in the Gulf.
The Iranian government said the length of detention of the mainly Indian crew on Stena Impero depended on the degree of their cooperation. “We are required by regulations to investigate the issue,” Allahmorad Afifipour, the head of ports and maritime in Hormozgan province, told Iranian state TV.
Afifipour said all 23 crew members from the ship were “safe and in good health”. The ship’s owners have asked to visit the crew.
Iranian MPs meeting on Sunday welcomed the ship’s capture and made calls for the government to consider imposing tolls on all British and US shipping heading to the Strait, a move that would lead to naval confrontations since the strait is considered international waters.
“It is required that the Iranian ship would be released as soon as possible so that we would not have to impose tolls on UK and US ships crossing the Strait of Hormuz,” said Mostafa Kavakebian, a member of the Iranian parliament’s national security committee.
Iranian officials were explicit that the capture on Friday was in retaliation for the capture of Grace 1. The speaker of Iran’s parliament, Ali Larijani, said the British “stole and got a response” from the Revolutionary Guards.
Britain privately admits it worked in conjunction with the US in the capture of Grace 1 but says it had independent intelligence that the ship contained oil bound for Syria, in violation of EU sanctions. It says the ship could not have been stopped had it not entered Gibraltarian waters.
It is possible ministers underestimated the reaction, particularly since Iran did not attempt any reprisal when the British – unlike the French or Germans – blamed the Revolutionary Guards for an attack on two tankers last month.
One Whitehall source admitted the government had considered turning a blind eye, admitting that an unworthy part of him had thought: “Let this cup pass from my lips.”
Ellwood again demanded the release of the Stena Impero, a call echoed by Oman in whose waters the ship was captured.
Speaking on Sky News, Elwood also made a stark call for a reversal of defence cuts. He said: “If we want to continue playing a role on the international stage – bearing in mind that threats are changing, all happening just beneath the threshold of all-out war – then we must invest more in our defence, including our Royal Navy. Our Royal Navy is too small to manage our interests across the globe if that’s our future intentions and that’s something the next prime minister will need to recognise.”
Britain has been nervous of the rules of engagement of the US’s long-touted plan for an international maritime security force, nicknamed Operation Sentinel. There is already a UN-endorsed anti-piracy operation off the coast of Somalia, while combined taskforces already operate in the Gulf, dedicated to security and fighting drug-smuggling.
There are also issues about the funding of any new larger operation proposed by US Central Command. But, most of all, Britain fears US-led convoys in the Gulf could by accident turn from something passive and defensive into something more volatile.
Javad Zarif, Iran’s foreign minister, said on Sunday that only “prudence and foresight” could alleviate tensions between his country and Britain. He said the US national security adviser, John Bolton, “having failed to lure Donald Trump into war of the century … is turning his venom against the UK in hopes of dragging it into a quagmire.”
Iran’s ambassador to the UK, Hamid Baeidinejad, said on Twitter that Britain needed to contain “those domestic political forces who want to escalate existing tension between Iran and the UK well beyond the issue of ships. This is quite dangerous and unwise at a sensitive time in the region,” he said, adding that Iran “is firm and ready for different scenarios.”
There is an awareness in Iranian circles that the a Johnson premiership could lead to the UK weakening its support for the nuclear deal from which Trump withdrew last year.
In the only cooling of tensions on Sunday, Iran said it appreciated Saudi Arabia’s efforts in the return of an Iranian ship that docked at Jeddah port because of technical problems in May.
“Iran appreciates efforts by the authorities of Saudi Arabia, Switzerland and Oman to secure the safe return of Iran’s Happiness 1 oil tanker,” said a foreign ministry spokesman, Abbas Mousavi.
Iranian media reported in early July that Saudi Arabia, Iran’s regional rival, was not allowing the ship to leave Jeddah because of a dispute over the payment of repair costs.