Britain will today join the US in warning that any action by Iran to close the Strait of Hormuz will be “illegal and unsuccessful”, insisting that western navies operating in the Gulf region must ensure the free flow of oil trade.

Amid signs that the European Union has agreed in principle to impose a ban on oil imports from Iran later this month, Philip Hammond, UK defence secretary, will warn the Iranians that Britain will respond militarily if Tehran attempts to close the strait in retaliation for sanctions.

As he prepares to hold his first meeting in Washington today with Leon Panetta, US defence secretary, Mr Hammond will say in a speech it is in the interests of all nations “that the arteries of global trade are kept free, opening and running”. He will add: “Disruption to the flow of oil through the Strait of Hormuz would threaten regional and global economic growth. Any attempt by Iran to do this would be illegal and unsuccessful.”