Iran's nuclear ambitions could plunge the world into "a new Cold War" with the Middle East, Foreign Secretary William Hague has warned.
He predicted a nuclear arms race among rival Middle Eastern states that would carry the dangers without the safety mechanisms of the old rivalry between the West and the USSR.
In an interview with The Daily Telegraph, he insisted Britain did not back military action against the Islamic republic - as Israel is thought to be planning.
But he said there was a "crisis coming down the tracks" that could result in "disaster" for world affairs.
"(The Iranians) are clearly continuing their nuclear weapons programme," Mr Hague said.
"If they obtain nuclear weapons capability, then I think other nations across the Middle East will want to develop nuclear weapons.
"And so, the most serious round of nuclear proliferation since nuclear weapons were invented would have begun with all the destabilising effects in the Middle East. And the threat of a new Cold War in the Middle East without necessarily all the safety mechanisms. That would be a disaster in world affairs."
There are growing fears that President Mahmoud Ahmadinejad will not be deterred by sanctions, the preferred route of the US and Britain to pressure the regime into abandoning its nuclear weapons programme.
Mr Hague stressed that "all options must remain on the table" concerning Iran but said that a military attack would have "enormous downsides". He added: "We are not favouring the idea of anybody attacking Iran at the moment."
Shadow foreign secretary Douglas Alexander said: "The risks of Iran obtaining nuclear weapons are real. Instead of raising the rhetoric, the Government should be focused on redoubling their efforts to increase the diplomatic pressure on Iran and find a peaceful solution to the issue."
