At its narrowest point, the Strait of Hormuz is just 21 miles wide – a thin, U-shaped stretch of water allowing access to the Persian Gulf, but which almost seals it off completely from the wider world.

A fifth of all global oil and a third of the world’s liquefied natural gas passes through the strait. And it is amid these busy shipping lanes that the potentially disastrous conflict between Iran and the United States is playing out. Now Britain has been dragged in, too.

On Friday, the Iranians seized a British-flagged Swedish-owned ship, the Stena Impero, diverting it to their own waters on the eastern side of the channel. The circumstances are murky. The Iranians claim the Stena Impero had been using the wrong lane in its journey to Saudi Arabia.

Iran’s hardline Islamic Revolutionary Guard Corps (IRGC) commanders know that their most powerful card is the most direct: to threaten the passage of oil and gas through the busy Strait of Hormuz, where the shipping lanes are only two miles wide

The ship’s location remains unknown and the crew have disappeared. The dangers presented by this latest episode are all too clear, however: they are another step on the road to a crippling world recession if not a catastrophic military confrontation.

The crisis in the Gulf is almost entirely the result of President Trump’s decision to sabotage the 2015 nuclear deal with Iran – a major achievement not just of his predecessor President Obama but five major powers, including the UK, which were fellow signatories. Trump withdrew even though Iran had scrupulously observed the letter of that accord.

In fact, according to leaked cables from Britain’s former Ambassador to Washington Sir Kim Darroch – reported in this newspaper – Trump pulled out largely to spite Obama.

The result is that America has re-imposed stringent sanctions on Iran, cutting it out of a banking system dominated by the dollar and making it almost impossible for Iran to sell its oil. Exports have collapsed from three million barrels a day to about 400,000, plunging the nation into a deep economic crisis. Tehran is now in a race against time to force America to lift the sanctions before the economy collapses altogether.

The crisis in the Gulf is almost entirely the result of President Trump’s decision to sabotage the 2015 nuclear deal with Iran

There are several measures at Iran’s disposal. It has launched cyber-attacks against the computer systems used by Saudi Arabian companies to pump crude oil. Iran’s proxies in Yemen, the Houthi, have attacked an emergency land pipeline that the Saudia Arabians can use to bypass Gulf shipping lanes.

But Iran’s hardline Islamic Revolutionary Guard Corps (IRGC) commanders know that their most powerful card is the most direct: to threaten the passage of oil and gas through the busy Strait of Hormuz, where the shipping lanes are only two miles wide.

The strait has correctly been described as a choke point – and choking is very much what gas and oil producers in the Gulf now face. The IRGC has already sabotaged six ships moored in Gulf harbours by attaching mines to their hulls. It used a missile to shoot down one of America’s 35 Global Hawk surveillance drones. And now it has seized a tanker flying the British flag.

UK ships have been told to stay away and others will do the same. But remember this: 85 per cent of Gulf oil is exported to Asia – China, India, Japan, Singapore and South Korea – and without that oil these states would be plunged into economic crisis, with knock-on effects for the entire global economy.

As, day by day, the tension rises, so does the risk of armed conflict – perhaps as the result of a misunderstanding or an accident. While Trump realises that his voters do not want another American-led war in the Middle East – the recent ones in Iraq and Afghanistan will have cost $5 trillion by about 2040 when the bills for injured veterans come in – some of the hawks around him do. Trump has already felt obliged to bolster the US Navy in the region, to shoot down an Iranian drone and to move 500 troops to Saudi Arabia.

The strait has correctly been described as a choke point – and choking is very much what gas and oil producers in the Gulf now face. The IRGC has already sabotaged six ships moored in Gulf harbours by attaching mines to their hulls. It used a missile to shoot down one of America’s 35 Global Hawk surveillance drones. And now it has seized a tanker flying the British flag

Now, with the hijacking of the Stena Impero, Britain finds itself enmeshed, and here the story gets still stranger. Earlier this month, Royal Marines boarded a large Iranian tanker, the Grace 1 at Gibraltar, claiming it was transporting crude oil to be refined in President Assad’s Syria. This delivery of oil was said to be in breach of EU sanctions.

The problem here is that Iran is not an EU member and so no EU sanctions apply to it. Did Britain act at the prompting of America?

It is a matter of urgency to discover which Minister in London took it upon themselves to order such an operation, since it was almost bound to elicit an Iranian response.

We had certainly been warned. A Royal Navy frigate in the Gulf had already warded off Iranian swift boats trying to board a tanker owned by BP. Why, then, was the Stena Impero sailing without an escort into such bitterly contested waters? Why, at a time of such tensions, are most British warships six weeks’ sailing time away and why are there so few of them?

It is a catalogue of failures.

On Tuesday , this crisis will drop into the lap of a completely new prime minister, almost certainly Boris Johnson, whose record of dealing with Iran as Foreign Secretary went no further than issuing ill-judged remarks about jailed mother Nazanin Zaghari-Ratcliffe that have helped keep her behind bars.

A deal offering some sort of trade-off between the impounded Grace 1 and the Stena Impero is now an absolute priority for the new PM. While we are about it, the UK should reimburse Tehran the £400 million it owes for Chieftain tanks which Iran paid in the days of the Shah. We took the money but did not deliver the tanks.

Beyond that, however, the new government in London must decide whether it wishes to act as America’s stooge in the Gulf. Right now, the UK has little diplomatic leverage in Washington but we should be urging Trump and Iran to engage in direct talks.

These must involve the seemingly emollient President Rouhani and smooth foreign minister Mohammad Javad Zarif. But such talks must also include the men with real power: Supreme Leader Khamenei and Qassem Soleimani, head of the IRGC Quds Force, responsible for intelligence and special operations.

We should be urging the Asian nations that depend upon Gulf oil exports to open their bulging wallets to help protect these vital shipping routes. What responsibility has been shouldered by Japanese Prime Minister Shinzo Abe, Indian premier Narendra Modi or Xi Jinping, the President of China? The UK has somehow found itself at the heart of the crisis even though we don’t import much Gulf oil.

If America is foolish enough to attack, Iran will punch back – and with serious consequences for the region and the wider world. For an economy as precariously poised as ours, a huge hike in oil and gas prices would be catastrophic.

We should remember that British diplomats and Ministers played a distinguished role in devising the nuclear deal which Trump so recklessly abandoned. And there is now another deal to be made, certainly to judge from recent talks between Iranian officials and Kentucky Senator Rand Paul.

The one conspicuous dove in Trump’s circle, Paul believes Iran might well be willing to agree to a permanent, rather than time-limited, regime of nuclear inspections in return for sanctions relief. Facing re-election next year by a war-weary electorate, Trump must be urged to go for it with both hands.