This would stop Iran from selling 450,000 barrels to Europe every day, potentially depriving the country of 30 per cent of its oil exports. Even if Iran found alternative buyers, it would probably have to cut the price of its crude, thereby losing billions of dollars.

This possible threat to its principal source of revenue has already drawn a belligerent response from Iran, which has warned that it would respond by blocking the Strait of Hormuz, the most important link in the global oil supply chain.

On Tuesday Iran criticised the US for sending an aircraft carrier, the USS John C. Stennis, through the Strait and cautioned America not to repeat this move.

"We recommend to the American warship that passed through the Strait of Hormuz and went to Gulf of Oman not to return," said General Ataollah Salehi, commander of Iran's regular armed forces.

Last month, EU foreign ministers failed to agree an oil embargo, largely because of objections from Greece, Italy and Spain, who together buy most of Iran's crude exports to Europe.

But Alain Juppe, the French foreign minister, signalled a renewed drive to secure a ban.

"Iran is pursuing the development of its nuclear arms, I have no doubt about it," Mr Juppe told French television, adding that France had responded with a unilateral decision not to buy Iranian oil. "We want the Europeans to take a similar step by January 30 to show our determination," said Mr Juppe.

Any such embargo would amount to "biting sanctions in the real sense of the word," said Mark Fitzpatrick, a director at the International Institute for Strategic Studies. But Iran's threat to retaliate by closing the Strait of Hormuz was "bluster", he added.

"That would invite a military attack and prevent Iran's own oil from going through the Strait," said Mr Fitzpatrick.

Other observers take a different view of Iran's threats. Nigel Kushner, chief executive of Whale Rock, a legal practice specialising in international trade and sanctions, said that Iran's warning to close the Strait of Hormuz should be taken seriously.

"The Iranians would find it difficult not to take quite drastic action if the EU does ban their oil imports," he said.

The Tehran regime, riven by factionalism and infighting, has found it impossible to agree a coherent response to its increasing isolation.

"Everyone is underestimating the internal political differences in Iran today," said Mr Kushner. These divisions, he added, made it more likely the regime would retaliate for any EU embargo.