Iran's Revolutionary Guard has brutally taunted the UK by hoisting the nation's flag over a seized British oil tanker and calling its beloved Royal Navy "impotent" — as tensions between the two escalate.

State TV footage shows Iranian armed forces on-board the Stena Impero after it was ambushed by masked troops who rappelled from helicopters to seize it in the Strait of Hormuz — a vital oil trade route — on Friday.

To add insult to injury, Iran's state TV also mocked the UK's Royal Navy in an article headlined "From piracy to impotence".

"Diplomatic spats aside, any objective review of the facts will not fail to recognise that in recent events in the Persian Gulf and the Gibraltar Strait, the UK has engaged in illegal and destabilising, activities on at least three occasions," the article reads.

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Foreign Secretary Jeremy Hunt spoke to reporters after an emergency government meeting about the "totally and utterly unacceptable" interception of the Stena Impero and "measures that we are going to take" to guarantee British vessels safe passage.

Hunt said that while speaking with Iranian Foreign Minister Javad Zarif, he again rejected Iran's assertion that Friday's incident reciprocated for Royal Marines taking part in the July 4 seizure of an Iranian tanker.

The taunts come as a new audio recording of the incident on Friday shows how Iranian forces ordered a British-flagged tanker to change course.

In the audio recordings, the Iranians tell the Stena Impero: "You are ordered: Change your course to three six zero. Three six zero degrees immediately. If you obey, you will be safe."

The frigate HMS Montrose (F236), in the Gulf to provide maritime security to merchant shipping, tells the Stena Impero: "As you are conducting transit passage in a recognised international strait, under international law your passage must not be impaired, impeded, obstructed or hampered."

Communication is established between HMS Montrose and the Iranians. They tell the British warship: "Foxtrot 236 this is SEPAH navy patrol boat. No challenge is intended. No challenge is intended. I want to inspect the ship for security reason."

HMS Montrose replies: "SEPAH navy patrol boat, this is British warship Foxtrot 236. Your requests that you are transmitting to the Stena Impero … hinder and impede her passage. You must not impair, impede, obstruct or hamper the passage of the MV Stena Impero.

"Please confirm that you are not intending to violate international law by unlawfully attempting to board the MV Stena."

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The message from the Iranians to the Stena Impero is repeated: "If you obey, you will be safe."

A speedboat of the Iran's Revolutionary Guard moves around a British-flagged oil tanker Stena Impero, which was seized on Friday by the Guard, in the Iranian port of Bandar Abbas. Photo / AP
A speedboat of the Iran's Revolutionary Guard moves around a British-flagged oil tanker Stena Impero, which was seized on Friday by the Guard, in the Iranian port of Bandar Abbas. Photo / AP

Britain's Ministry of Defence confirmed the recording was genuine.

Video footage released by Iran's Islamic Revolutionary Guard Corps on Saturday showed a ship with the tanker's markings being surrounded by speedboats before troops in balaclavas descend onto the vessel by rope from a helicopter.

Little clue has been given by Britain on how it plans to respond to the attack — which is understood to be a retaliation for the British capture of an Iranian tanker two weeks earlier.

The British Government is expected to announce its next steps in a speech to parliament on Monday.

But experts on the region say there are few obvious steps London can take at a time when the US has already imposed the maximum possible economic sanctions, banning all Iranian oil exports worldwide.

"We rant and rave and we shout at the ambassador and we hope it all goes away," said Tim Ripley, a British defence expert who writes about the Gulf for Jane's Defence Weekly.

"I don't see at this point in time us being able to offer a concession that can resolve the crisis. Providing security and escort for future ships is a different matter."

A day after calling the Iranian action a "hostile act", top British officials kept comparatively quiet on Sunday, making clear that they had yet to settle on a response.

"We are going to be looking at a series of options," junior Defence Minister Tobias Ellwood told Sky News. "We will be speaking with our colleagues, our international allies, to see what can actually be done.

"Our first and most important responsibility is to make sure we get a solution to the issue to do with the current ship, make sure other British-flagged ships are safe to operate in these waters and then look at the wider picture."

The Iranian capture of the ship in the global oil trade's most important waterway is the latest escalation in three months of spiralling confrontation with the West that began when new, tighter US sanctions took effect at the start of May.

A speedboat and a helicopter of the Iran's Revolutionary Guard move around a British-flagged oil tanker Stena Impero. Photo / AP
A speedboat and a helicopter of the Iran's Revolutionary Guard move around a British-flagged oil tanker Stena Impero. Photo / AP

Britain was thrust more directly into the confrontation on July 4 when its Royal Marines seized an Iranian tanker off the coast of Gibraltar. Britain accused it of violating sanctions on Syria, prompting repeated Iranian threats of retaliation.

Iran's official line is its capture of the Stena Impero was because of safety issues, but it has done little to hide the move was retaliatory.

The tactics it used — with masked troops rappelling from helicopters — matched those the British used two weeks before.

Parliament speaker Ali Larijani spelled it out more clearly on Sunday, telling a parliament session: "The Revolutionary Guards responded to Britain's hijacking of the Iranian tanker." Iran's Foreign Minister Mohammad Javad Zarif blamed Washington and US President Donald Trump's hawkish national security adviser John Bolton for luring Britain into conflict.

"Having failed to lure @realDonaldTrump into War of the Century … @AmbJohnBolton is turning his venom against the UK in hopes of dragging it into a quagmire," Mr Zarif wrote on Twitter. "Only prudence and foresight can thwart such ploys."

In a letter to the UN Security Council, Britain said the Stena Impero was approached by Iranian forces in Omani territorial waters, where it was exercising its lawful right of passage, and the action "constitutes illegal interference".