Two-way radio exchanges have emerged between a Royal Navy frigate and Iranian armed forces vessels.
Among other comments the controller of the Iranian vessel can be heard saying: "If you obey you will be safe, if you obey you will be safe, alter your course to 360 degrees immediately, over."
That was just moments before the Stena Impero was seized in the Gulf on Friday.
This latest incident follows the seizure of a British-flagged oil tanker and the detention of Iran's Grace 1 oil tanker in Gibraltar earlier this month. Both the UK and Iran have blamed each other for the rise in diplomatic tensions.
In a tweet, Iran's foreign minister, Javid Zarif, said Iran guaranteed the security of the Gulf and the Strait of Hormuz.
But he added that the UK must cease being an accessory to what it called the "Economic Terrorism" of the US.
Tehran says the crew of the Stena Impero are in good health. The issue has become a big talking point in Iran.
"Our country has now reached such pride and strength that it can respond to all evil acts that Britain is carrying out in public," said one Iranian citizen, Ali Dosarti.
"This is a source of pride for us. We're proud of that."
A map has now been released by the UK's Department for Transport, saying the Stena Impero was in Omani Territorial Sea when it was intercepted.
This new information will add weight to comments by the UK foreign secretary, Jeremy Hunt that Tehran's actions showed, what he called: "Worrying signs Iran may be choosing a dangerous path of illegal and destabilising behaviour."