Britain risks being dragged into wider confrontation against Iran as highest-level security alert issued

Analysis: What happened with the British tanker, according to military and diplomatic sources, was not an obvious attempt to attack the ship, but raising the scale of harassment and brinkmanship

Accusations and recriminations over claims that Iran tried to impede a British ship in the Straits of Hormuz risk drawing this country into a widening confrontation in one of the busiest oil supply routes in the world.

The escalation in the tension was underlined when British ships in the Gulf were put on the highest state of alert. This means dangerous encounters may be imminent.

The level three warning was issued the day before a Royal Navy warship intervened and trained its guns on Iranian gunboats that tried to impede the path of a British tanker.

The belligerent standoff between Tehran and the Trump administration since earlier attacks on tankers, and the shooting down of an American drone by the Iranians, has been more about geopolitics.

Iran and the US have been on a collision course over president Donald Trump pulling out of the agreement the country had with international powers over its nuclear programme, and fears in Tehran that the US administration was seeking to enforce regime change.

What happened with the British tanker, according to military and diplomatic sources, was not an obvious attempt to attack the ship, but raising the scale of harassment and brinkmanship.

The ship, British Heritage, was near the island of Abu Musa, which is in disputed waters, but HMS Montrose, the British frigate which moved in to head off the gunboats of the Iranian Revolutionary Guards Corps (IRGC) stayed in international seas.

This incident is more about oil and, technically, not strictly just about Iran, but also brings in Syria. The background was the seizure by Royal Marines of an Iranian-owned tanker, Grace I, off Gibraltar on charges that it was breaking European Union sanctions imposed on the Assad regime.

It was, however, the first time a European state has interdicted a tanker for Syria on the seas and, despite denials from London, Tehran was convinced that the action was taken on US instructions.

It was a view which was strengthened by Trump’s national security advisor, John Bolton, one of those who had advocated regime change, tweeting what happened was “excellent news”, adding “America & our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade.”

The message from London was the need to calm the situation.

But a senior Iranian official, an advisor to Grand Ayatollah Khameini, threatened that a British tanker would be captured if Grace I was not released. The British Ambassador was summoned and the UK accused of a “form of piracy”.

The Iranians have demanded the release of the crew of the tanker – mainly Indians and Pakistanis and a few Ukrainians. They have also demanded the release of the ship, which they insist was not breaking sanctions, and its cargo.

This is not just a matter of principle. The Trump administration’s sanctions are targeting Iran’s oil exports, the country’s main revenue earner, with the stated aim of crippling the economy.

Foreign secretary Jeremy Hunt yesterday said the tension in the Gulf made the case for strengthening the Royal Navy. The Tory leadership hopeful said the UK had “run down the navy too much” in recent decades. Mr Hunt, the son of an admiral, promised to add more warships to the fleet if he entered No 10.

Writing in the Daily Telegraph, Mr Hunt said: “Three vessels from a foreign military power tried to seize a British ship conducting its rightful business.

“The simplicity of these words belies the incredible menace behind Iran's actions. Not for the first time, Britain's interests were defended by the Royal Navy.”

The remaining signatories to the Iran nuclear deal, the Joint Comprehensive Plan of Action (JCPOA) Britain, France, Germany, Russia and China, estimate that Iran needs to sell around 700,000 barrels a day to pay its bills – it is now down to 200,000 thanks to American sanctions.

Grace I was fully loaded. It has the capacity to carry 2 million barrels of crude oil, which is lighter than fuel oil. The reason it seemed top heavy, according to reports, was that it was carrying fuel oil. And a tanker full of oil, crude, let alone fuel, is a lot of money for a badly hit economy to lose.

It would be impossible for the UK to release the cargo to the Iranians with the various sanctions in place, leaving open the possibility of more standoffs in the Gulf, with the Iranians insisting that the British action was illegal.

The Iranian reaction to what has happened has been to say nothing took place at all. Mohammad Javad Zarif, the foreign minister, accused the UK of making the claims “for creating tensions”.

Zarif is one of the key architects of the JCPOA, a reformist, and his statement is likely to have been for domestic consumption. The IRGC said it “denied claims by American sources” that it tried to seize the ship.

The IRGC’s protestations may be treated with scepticism, but it is fact that it is the US that is keen to play up the incident while the British government had adopted a relatively restrained tone.

A government spokesman said: “Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz. We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region.”

The Royal Navy currently has a frigate, four mine-hunters and a Royal Fleet Auxiliary support ship stationed near the Gulf. Continuing rise in tension may mean another warship will have to be dispatched, this will mean adding to the already enlarged international military hardware with the risk of sparking violence – the law of unintended consequences; a scenario British government insists it wants to avoid.