LONDON     -   A recording has emerged of radio exchanges between a Royal Navy frigate and Iranian armed forces vessels, moments before a UK-flagged oil tanker was seized in the Gulf on Friday.

In the recording, an Iranian vessel tells HMS Montrose it wants to inspect the Stena Impero for security reasons.

Chancellor Philip Hammond said the government would pursue “every possible diplomatic route” in response.

But Iran’s ambassador to London warned the UK against escalating tensions. Hamid Baeidinejad said in a tweet: “This is quite dangerous and unwise at a sensitive time in the region. Iran however is firm and ready for different scenarios.” The UK’s foreign secretary has urged Iran to reverse the tanker’s “illegal” seizure.

Jeremy Hunt said MPs would be updated on Monday about what “further measures” the government would take.

On Friday, the Stena Impero was seized by the Iranian Revolutionary Guard in the Strait of Hormuz, a key shipping route in the Gulf.

A second British-linked tanker, the MV Mesdar, was also boarded by armed guards but was released.

Tehran said the Stena Impero was “violating international maritime rules”.

Video released by Iran’s Revolutionary Guard-affiliated Fars news agency on Saturday appeared to show the moment the tanker was raided. It shows masked forces dropping down ropes on to the ship from a helicopter after it was surrounded by high-speed vessels.

HMS Montrose was alerted but it was too far away to stop the seizure.

Iran’s state-run IRNA news agency said the tanker was captured after it collided with a fishing boat and failed to respond to calls from the smaller craft.

But Foreign Secretary Jeremy Hunt said it was seized in Omani waters in “clear contravention of international law” and then forced to sail into Iran.

The tanker’s owners, Stena Bulk, said it had been complying with regulations and had been in international waters.

It said it had requested access to the port of Bandar Abbas to visit crew members, who are Indian, Russian, Latvian and Filipino, and said to be in good health. The seizure of the Stena Impero comes two weeks after Royal Marines helped seize Iranian tanker Grace 1 off Gibraltar, because of evidence it was carrying oil to Syria in breach of EU sanctions.

Mr Hunt said the Grace 1 was detained legally, but Iran said this was “piracy” and threatened to seize a British oil tanker in retaliation. 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.

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.

“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.

Defence minister Tobias Ellwood told Sky News sanctions against Iran were among the options being considered in response. 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”.

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.

Former Cabinet minister Iain Duncan Smith described the seizure of the Stena Impero as a “major failure” by the UK.

Speaking on the media, he called on the government to explain why it did not increase protection for ships in the region after he said the US had offered its support.

Iran’s foreign minister Javad Zarif tweeted that the UK “must cease being an accessory to #EconomicTerrorism of the US”.

He said Iran guarantees the security of the Gulf and the Strait of Hormuz, and insisted its action were to “uphold international maritime rules”.

The latest developments come against a backdrop of deteriorating relations between Iran and the UK and US.

Tensions between the US and Iran have risen since April, when the US tightened sanctions it had re-imposed on Iran after unilaterally withdrawing from a 2015 nuclear deal.

The US blamed Iran for attacks on tankers since May, which Tehran denies. On Friday, the US claimed to have destroyed an Iranian drone in the Gulf.

The UK government has remained committed to the deal, which curbs Iran’s nuclear activities in return for the lifting of sanctions tensions.

However, the UK’s decision to help seize the Iranian tanker Grace 1 earlier this month infuriated Iran.

Last week, Iranian boats attempted to impede a British oil tanker in the region before being warned off by HMS Montrose. Iran denied it was attempting to seize the ship.