Philip Hammond warned the regime in Tehran that the UK will not tolerate the ‘very significant consequences’ if it fulfils a threat to block the Strait of Hormuz.
He signalled that such action would be blocked by force of arms.
Mr Hammond used a speech in Washington to warn Iran that any move to close the key Gulf trade route would be opposed by the Royal Navy.
‘Any attempt by Iran to do this would be illegal and unsuccessful,’ he said in a speech at the Atlantic Council.
Then in a television interview, Mr Hammond said he wanted to send a ‘very clear message to Iran’ that the UK would not allow the Strait of Hormuz to be closed.
The Royal Navy operates mine clearance vessels in the Gulf as part of a joint taskforce based in Bahrain.
He said: ‘We are an integrated part of the naval taskforce in the Gulf and one of the missions of that taskforce is to ensure that those shipping lanes remain open.