Iran has threatened to close the Strait of Hormuz, a vital route for the oil trade, if the West widens sanctions against the country amid an ongoing row over Tehran's nuclear programme.
Vice President Mohammad Reza Rahimi said on Wednesday his country will not allow a "drop of oil" to pass through the strait if Western powers take fresh measures against the Islamic Republic.
His warning was underlined by the head of the Iranian navy, Admiral Habibollah Sayyari, who said it would be "really easy" for his forces to block the waterway through which a sixth of the world's oil flows.
But Britain's foreign office dismissed the threats as "rhetoric" intended to distract attention from its nuclear programme.
"Iranian politicians regularly use this type of rhetoric to distract attention from the real issue, which is the nature of their nuclear programme," a spokesman said.
The Strait of Hormuz links the Gulf oil-producing states of Bahrain, Kuwait, Qatar, Saudi Arabia and the United Arab Emirates with the Indian Ocean. Around 40% of the world's tanker-borne oil passes through them.
The US maintains a naval presence in the Gulf, primarily to ensure the oil routes remain open.
Tensions between Iran and the West escalated after a report last month by the International Atomic Energy Agency - the UN's nuclear watchdog - said Iran had carried out tests related to the "development of a nuclear device".
The US announced further economic sanctions, and the European Union is considering fresh measures targeting Iran's oil and financial sectors.
The moves prompted a furious Iranian backlash, with demonstrators storming the British embassy in Tehran chanting "death to England", apparently with the backing of the regime.
The Foreign Office spokesman said the government remained "extremely concerned" at the "possible military dimension" to Iran's nuclear programme.
"We want to find a negotiated solution to this issue and that is why we will continue to pursue a dual track strategy of pressure and engagement until Iran convinces the international community that it is not pursuing a military nuclear programme," the spokesman was quoted by the Press Association as saying.