The dramatic moment a British-flagged tanker was seized has been revealed in startling footage released by Iranian State TV.
Footage shows commandos in black ski masks on a helicopter before rappelling onto the British registered Stena Impero as it passed through the Strait of Hormuz yesterday afternoon.
A number of speedboats can also be seen surrounding the vessel, as the tanker was seized with a crew of 23 aboard. None of the crew are British nationals.
The footage comes as Foreign Secretary Jeremy Hunt said the seizing “raises very serious questions” about the security of British and international shipping on the Strait.
Mr Hunt, speaking after a meeting of the Government's emergency committee Cobra, said the vessel was seized in Omani waters in "clear contravention of international law".
He told reporters inside the Foreign and Commonwealth Office that, having spoken to his Iranian counterpart Mohammad Javad Zarif, Tehran saw the situation as a "tit for tat" following the detention of Grace 1 in Gibraltar.
"Nothing could be further from the truth," he said.
"Grace 1 was detained legally in Gibraltarian waters because it was carrying oil, against EU sanctions, to Syria, and that's why the Gibraltarian authorities acted totally with respect to due process and totally within the law.
"Stena Impero was seized in Omani waters in clear contravention of international law, it was then forced to sail into Iran.
"This is totally and utterly unacceptable. It raises very serious questions about the security of British shipping, and indeed international shipping, in the Strait of Hormuz."
Mr Hunt said MPs would be updated about what "further measures" the Government will take, on Monday, adding that the threat level had been raised to three.
"Our priority continues to be to find a way to de-escalate the situation.
"That's why I reached out to the Iranian foreign minister, that's why due process in Gibraltar continues.
"We need to see due process happening in Iran as well, we need to see the illegal seizing of a British flagged vessel reversed, we need that ship released, and we continue to be very concerned about the safety and welfare of the 23 crew members."
The Foreign Office earlier summoned the Iranian charge d'affaires, Mohsen Omidzamani, following the incident.
Iran has directly linked the seizure of the vessel with Britain's role in detaining a tanker carrying Iranian oil earlier this month.
A spokesman for Iran's Guardian Council was quoted as saying "the rule of reciprocal action is well known in international law" and that Tehran made the right decision in the face of an "illegitimate economic war and seizure of oil tankers".
The explanation, which contrasts with a suggestion on Friday night that the Stena Impero was "violating international maritime rules" and had collided with a fishing boat, came as the UK Government warned British ships to stay away from the Strait of Hormuz.
HMS Montrose, which is patrolling the Persian Gulf to protect shipping, and earlier this month intercepted Iranian patrol boats surrounding another UK-flagged tanker, reportedly arrived minutes too late to prevent the latest incident.
Defence Secretary Penny Mordaunt described the incident as a "hostile act" and told Sky News the frigate was 60 minutes away from being able to help.
Stena Bulk, which owns the Stena Impero, said the ship was in "full compliance with all navigation and international regulations" when it was seized by the Iranian Revolutionary Guard.
A second oil tanker, the Liberian-flagged Mesdar, which is managed by Norbulk Shipping UK, veered off course towards the Iranian coast after it was boarded by armed guards at about 5.30pm on Friday.
The Mesdar's Glasgow-based operator said communication had since been re-established with the ship and the crew were unharmed. The tanker was reportedly allowed to resume navigation.
Downing Street said Prime Minister Theresa May was in her constituency but was being kept informed of developments.
Former chief of defence staff Lord Richards said Britain was "pretty limited" in what military action it could take without the support of allies such as America, should economic sanctions fail to resolve the situation.
But Mr Hunt said he had spoken to US Secretary of State Mike Pompeo about the situation, and President Donald Trump said the US would be "working with the UK".
The Foreign Secretary said the US had a "different approach" to dealing with Iran, and that the UK would continue to support the Iran nuclear deal.
"This is about the safety and security of British and international shipping in one of the most important seaways in the world, and that is why we're calling on Iran to reverse this illegal act," he said.
"We're looking for ways to de-escalate the situation but we're also very clear that we will do what it takes to ensure the safety and security of British and international shipping."
France and Germany joined condemnation of Iran's actions, which have triggered concerns that it will lead to further oil price hikes amid heightened tensions in the Gulf involving Iran, the US and UK.
Fears had been growing that the Iranian authorities were trying to seize a UK ship in retaliation for the seizure of the Grace 1 tanker off the coast of Gibraltar on July 4 in an operation involving Royal Marines.
The Grace 1 was suspected of violating EU sanctions by carrying a cargo of Iranian oil destined for Syria in breach of sanctions and Mr Hunt later offered to help release the vessel if Iran guaranteed it would not breach sanctions imposed on Bashar Assad's regime.
But Iran's supreme leader Ayatollah Ali Khamenei called the tanker's seizure an act of "piracy" on Tuesday and warned the UK to expect a response.
A statement from Stena Bulk said ship manager Northern Marine Management had lost contact with the crew of 23 after "unidentified small crafts and a helicopter" approached the vessel at about 4pm on Friday.
Additional reporting by Press Association