Iran's nuclear ambitions could plunge the world into "a new Cold War" with the Middle East, William Hague has warned.
The Foreign Secretary 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," 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.
Israel is rumoured to be planning military strikes within months. Ahmadinejad claimed this week that Iran had loaded its first domestically-made fuel rod into a nuclear reactor.
Hague stressed that "all options must remain on the table" concerning Iran but said that a military attack would have "enormous downsides".
"We are very clear to all concerned that we are not advocating military action," he said.
"We support a twin-track strategy of sanctions and pressure and negotiations on the other hand."
He added: "We are not favouring the idea of anybody attacking Iran at the moment."