In the radio recording the Iranian vessel can be heard telling a ship – thought to be the Stena Impero – to change its course, saying: “If you obey you will be safe.”
HMS Montrose identifies itself in the recording, obtained by British maritime security firm Dryad Global.
It 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.”
The frigate then asks the Iranian vessel to confirm it is not “intending to violate international law” by attempting to board the tanker.
How did the UK react?
Speaking after a call with his Iranian counterpart on Saturday, Mr Hunt said Iran viewed this as a “tit-for-tat situation” but he added that “nothing could be further from the truth”.
Ministers have held emergency Cobra meetings and a senior Iranian diplomat was summoned to the Foreign Office.
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Media captionJeremy Hunt says Iran views the tanker seizure as a ‘tit-for-tat situation’
“Our priority continues to be to find a way to de-escalate the situation,” Mr Hunt said.
The government is advising UK shipping to stay out of the area.
Speaking to Sky News on Sunday, Mr Ellwood denied that the UK had taken its “eye off the ball” and said “it is impossible simply to escort each individual vessel”.
Asked if sanctions were off the table, Mr Ellwood said: “Cobra was taking place yesterday, so we’re looking at the operational responsibilities from that but, yes, we are going to be looking at a series of options.”
Labour shadow justice minister Richard Burgon told the same programme the UK should avoid becoming Donald Trump’s “sidekicks” and warned that a US-backed conflict with Iran could be worse than the Iraq War.
France and Germany called on the Iranian authorities to quickly release the Stena Impero.
The European Union’s foreign affairs office, which represents 28 member states, expressed “deep concern”.
How ‘British’ is the tanker?
Ships must fly the flag of a nation state, explains Richard Meade, managing editor of maritime intelligence publication Lloyd’s List.
But that doesn’t need to be the same nation as its owners, its crew, or its cargo, he says.
The Stena Impero is Swedish-owned and those on board are Indian, Russian, Latvian and Filipino.
But it’s the UK flag that is important symbolically, he says. “Historically speaking it means that the UK owes protection to the vessel.”
“The UK has political responsibilities to anything that is flagged. And that’s why it’s much more serious than if there just happened to be a British captain on board.”
He says the impact on trade in the region had so far been minimal, but warns that if the international community began viewing the Strait of Hormuz as a dangerous place, it could create a “very different” scenario.
The seizing of a British-flagged tanker in Omani waters, empty and inbound to a Saudi port, marks a serious escalation in a whole catalogue of recent incidents in the Gulf.
It comes on the back of the mysterious mining of tankers, the downing of both US and Iranian drones and the near capture of another British-flagged tanker only a few days ago.
Britain wants its response be two things: Measured and multinational.
The government is trying to send a robust message to Iran that this action is unacceptable, not just to the UK but to the rest of the world, but not so robust that it ends up being part of an avoidable US military strike.
This has become a highly volatile situation where not everyone believes in diplomacy. There are figures in Washington who have been pushing for an ever-tougher line with Iran.
And there are figures in Iran, notably in the Revolutionary Guards Corps and the security apparatus, who are quite prepared to push this right up to the brink of a conflict, yet probably stopping just short of one.