British ships operating in the Gulf have been put on the highest state of alert amid fears that UK-flagged commercial vessels are vulnerable to attack by Iranian gunboats.

The British government changed the alert to level 3 on Tuesday, a day before a Royal Navy warship trained its guns on Iranian gunboats that tried to disrupt the passage of an Isle of Man-flagged vessel. Level 3 is described as equivalent to the domestic security classification of critical in which there is an imminent risk of an incident.

The alert is for the 15-30 large British-owned vessels that travel through the Gulf every day.

The UK said the commercial ship British Heritage was in disputed waters claimed by Iran when it was challenged by three Iranian vessels and ordered to move north. HMS Montrose then intervened, aiming its guns at the Iranian boats to ward them off.

The UK, keen to de-escalate the crisis and preserve the international nuclear deal with Tehran, is reluctant to release the video of the incident, which was first reported by the US late on Wednesday night.

“It seems that the Brits and the French and the Germans are coming around to the idea that the Iranians are up to no good!” President Trump tweeted on Thursday.

Britain has one warship, HMS Montrose, and three minesweepers operating in the area and so lacks the capacity to escort all British-linked ships moving through the Strait of Hormuz. Defence officials said that the British deployment is kept under constant review, but it was not possible to escort every ship travelling in Gulf waters.

British officials promised the Royal Navy “will be resolutely defending British maritime interests in the Gulf, but has no interest in escalating the situation”.

The threat to British shipping in the Gulf follows the seizure by Royal Marine commandos of an Iranian-flagged vessel, Grace 1, off the Strait of Gibraltar last Thursday, that prompted Tehran to threaten reprisals. The tanker is suspected of breaching European Union sanctions by carrying a shipment of Iranian crude oil to Syria.

The Indian captain of the ship and its chief officer were formally arrested on Thursday by Gibraltarian authorities after a week-long investigation. The ship remains detained and Iran has denounced the actions as piracy. Documents and electronic equipment was seized from the vessel, a spokesperson said.

British Heritage had been due to collect oil in Basra, Iraq, but abruptly turned round on Saturday on the basis of British government advice. It was seeking to go through the crowded waters without a load when it faced an Iranian instruction to go clearly into Iranian waters.

The waters in which British Heritage was accosted are claimed by Iran and by the United Arab Emirates. It is not possible for British shipping to go through the straits without entering Iranian waters.

Q&A Why is the Gulf of Oman so important for shipping oil? Show The strait of Hormuz, which provides passage from the Gulf of Oman to the open sea, is the most important gateway for oil exports in the world. With Iran on its northern shore, and the UAE and Oman on its southern shore, the US Energy Information Administration (EIA) calls it the world’s worst 'chokepoint' In 2016, 18.5m barrels of crude oil were transported each day through the strait of Hormuz, compared with 16m through the strait of Malacca, which runs between the Indonesian island of Sumatra, Malaysia and Thailand, connecting the Indian Ocean with the South China Sea. 5m barrels of crude oil are transported annually through the next largest chokepoint, the Suez canal. Phillip Inman

Speaking about the latest confrontation, the Ministry of Defence said in a statement: “Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the strait of Hormuz.”

HMS Montrose was “forced to position herself between the Iranian vessels and British Heritage and issue verbal warnings to the Iranian vessels, which then turned away,” the statement said. “We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region.”

Iran has rejected the claims. The foreign minister, Javad Zarif, called the British allegations “worthless” in remarks to the semi-official Fars news agency. “Apparently the British tanker has passed. What [the British] have said themselves and the claims that have been made are for creating tension and these claims have no value,” Zarif said.

Iran’s Islamic Revolutionary Guard Corps also issued a statement denying involvement.

The UK is waiting to hear detailed proposals from the US about a new international maritime security operation across the Gulf but ministers appear to be nervous about the implications of the plans, pointing to the existing 33-nation coalition on shipping.

Timeline Recent tensions in the Gulf Show The UAE says four commercial ships off its eastern coast 'were subjected to sabotage operations'. Yemen's Houthi rebels launch a drone attack on Saudi Arabia, striking a major oil pipeline and taking it out of service. Saudi Arabia subsequently blames Iran for the attack. A rocket lands near the US embassy in the Iraqi capital, Baghdad, without harming anyone. It's not clear who is behind the attack, but after the initial reports, Donald Trump tweets: 'If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!' Saudi Arabia says 26 people were wounded in an attack by Yemen's Houthi rebels on an airport in the kingdom's south-western town of Abha. Two oil tankers near the strategic strait of Hormuz were reportedly attacked in an assault that left one ablaze and adrift. 44 sailors were evacuated from both vessels and the US navy assisted. Iran’s Revolutionary Guards say they have shot down what they called a US 'spy' drone they claim was flying in in the country’s airspace. The US military confirm one of its drones has been taken down, but say it was in international airspace. Donald Trump reportedly gives approval for the US military to launch strikes on Iran in retaliation for the loss of the drone, before pulling back at the last minute. The Iranian and US presidents trade insults, with Hassan Rouhani suggesting that Donald Trump suffered from a “mental disorder” and Trump once more threatening Iran with “obliteration”. Iran summons UK ambassador over an incident off Gibraltar as Royal Marines seize a tanker, Grace 1, the UK suspects of carrying oil to Syria. The International Atomic Energy Agency, the body tasked with verifying Iranian compliance with the terms of the nuclear deal, verifies that Tehran has breached the agreed 3.67% limit for enriched uranium. The UK government says three Iranian boats were warned off by the frigate HMS Montrose after Iranian boats 'attempted to impede' a British oil tanker in strait of Hormuz. Tehran denies involvement. In a major escalation, Iran seizes the Stena Impero, a British-flagged tanker, off its coast. Iranian officials later make it clear that the capture was in retaliation for the capture of the Iranian supertanker Grace 1 earlier in July. Despite US attempts in the courts to prevent it, Gibraltar says it will free oil tanker at centre of the Iran row. Iran gives assurances the oil is not destined for Syria, where selling it would breach international sanctions against Britain accuses Iran of breaching those assurances after Tehran acknowledged the oil had been sold, and the reflagged tanker Adrian Darya 1, previously known as Grace 1, had reached its final destination after the ship was photographed off the coast of Syria.

The US proposal is likely to include a requirement for Asian countries to do more financially to subsidise the security operation, with Washington pointing out that a huge proportion of oil travelling through the Gulf is destined for Asian markets.

Jeremy Hunt, the foreign secretary, appeared to recognise the difficulty the UK faced in protecting its maritime shipping due to successive defence budget cuts. He tweeted: “Latest incident with Iranian boats in the Gulf shows exactly why we must invest more in defence. We have run down our Navy too much and this must now be urgently reversed. We need to show confidence in the world by putting our money where our mouth is!”

Tehran has been accused of sabotaging oil tankers in the Strait of Hormuz in an attempt at economic retaliation against the US after Donald Trump pulled out of a 2015 nuclear agreement and imposed crippling sanctions on Iran.

Washington and Tehran came close to direct military conflict last month, when Iran shot down a US drone and Trump ordered retaliatory airstrikes, only to call them off minutes beforehand.

The UK and other European countries have been trying to save the deal, in which Tehran agreed to limits on its nuclear programme in exchange for relief from international sanctions.

Russia and China, both signatories to the nuclear agreement along with the UK, France and Germany, called for restraint. The Kremlin spokesperson Dmitry Peskov said “freedom of navigation should be ensured in the Persian Gulf and the Strait of Hormuz”.