Iran's foreign minister has accused the United States of trying to drag Britain into a quagmire over a tanker crisis in the Gulf.
Newly released audio reveals the tense exchange between the Montrose and the Iranian military in the Strait of Hormuz last Friday.
British navy frigate HMS Montrose was alerted but it was too far away to stop the seizure.
For weeks Iran had threatened to seize a British tanker in retaliation for the United Kingdom's capture of an Iranian tanker two weeks earlier in Gibraltar.
British Foreign Secretary Jeremy Hunt says Iran has violated international law.
"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 and so we spent a long time this afternoon in COBRA discussing how we can guarantee the security of British and international shipping."
