The British government raised its security warning for shipping in Iranian waters to its highest level as the Royal Navy was forced to fend off the attempted obstruction of a British oil tanker by Iranian Revolutionary Guards. British flagged ships were notified at the beginning of this week that Iranian waters were considered a level three, or “critical” security environment, the Telegraph understands. The security alert, which amounts to advice to avoid Iranian waters where possible and would have been accompanied by advice on specific precautions to take, came after Iran threatened “reciprocal” action for the recent seizure of an Iranian tanker by the Royal Navy near Gibraltar. That action appeared to come on Wednesday, when the British Heritage, owned by BP Shipping and registered to the Isle of Man, was approached by three Iranian Islamic Revolutionary Guard Corps boats as it sailed through Persian Gulf towards the Strait of Hormuz. The Iranians ordered the vessel to stop in nearby Iranian territorial waters, according to the Ministry of Defence, but withdrew after HMS Montrose, a Royal Navy frigate which had been escorting the tanker, aimed its guns on the Iranians and warned them to move away. A Ministry of Defence spokesperson said: “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 tanker is believed to have been in waters disputed by Iran and the United Arab Emirates when the incident took place A US aircraft flying overhead filmed the incident, but the footage had not been released by late Thursday evening. Read more The incident is the most serious involving a British ship since the current crisis in the Gulf began and will raise fears that Britain could get dragged into a brewing military confrontation between the United States and Iran in the Gulf. Iran warned last week that it might seize a British oil tanker after Royal Marines boarded and detained the Grace 1, a super tanker carrying two million tons of Iranian oil, as it passed through Gibralatan waters last Thursday. British and Gibraltan authorities say they suspect the ship of running oil to Syria, in breach of EU sanctions, and have denied the move was targeted at Iran. Police in Gibraltar said on Thursday that they have arrested and interviewed under caution the Grace 1's captain and chief officer on suspicion of breaching EU sanctions on Syria, in a move likely to further inflame tensions. Neither man has yet been charged. Iran's revolutionary guard denied attempting to seize the British Heritage, but a senior commander also warned that Britain would "strongly regret" detaining the Grace 1 . "If the enemy had made the smallest assessment they wouldn't have done this act," said Rear-Admiral Ali Fadavi on Thursday. "Our reciprocal action will be announced." Iran earlier described the seizure of Grace 1 as an act of "piracy" and accused Britain of bowing to US pressure to hinder its attempts to export oil under the terms of the 2015 nuclear deal. Gulf of Oman Mohsen Rezaei, a general in the Revolutionary Guard Corps and an advisor to Iran’s supreme leader, on Friday warned Iran might go after a British tanker. And President Hassan Rouhani called the seizure of the vessel "mean and wrong" during a Cabinet meeting on Wednesday. He warned London: "You are an initiator of insecurity and you will understand its repercussions," without elaborating. Oil companies have warned that continued disruption around the Strait of Hormuz, which handles up to a third of the world's seaborne oil exports, could have a dramatic impact on petrol prices. There are usually between 15 and 30 British flagged ships operating in the vicinity of the strait at any given day, more than the Royal Navy can realistically provide escorts for. Besides HMS Montrose, a type 23 frigate, the Royal Navy has four minesweepers and the Royal Fleet Auxiliary ship Cardigan Bay in the region. Britain is also a member of a 33-nation Combined Maritime Force with a mandate to protect shipping in the Western Indian Ocean from piracy and terrorism. The United States has said it wants a new international force to guarantee freedom of navigation in the Persian gulf, although it has yet to lay out those plans in detail. Last month Donald Trump called of military strikes against the Islamic Republic after it shot down a US surveillance drone over the Strait of Hormuz. Tensions in the Persian Gulf have risen dramatically since Donald Trump pulled the United States out of the Iran nuclear deal in May 2018 and imposed a series of punishing sanctions on the Islamic Republic, including measures designed to prevent it exporting oil. Britain, France, and Germany have defended the deal, including Iran's right to export oil, but Iran has complained the European powers have done too little to help it continue to trade. It began to violate the deal's restrictions on uranium enrichment in protest last week. A spokesman for BP, which operates the British Heritage tanker, said: "Our top priority is the safety and security of our crews and vessels. While we are not commenting on these events, we thank the Royal Navy for their support."