Iran's nuclear ambitions could plunge the Middle East into "a new Cold War", the UK foreign secretary has warned.
Interviewed by the Daily Telegraph, William Hague said if Iran developed nuclear weapons then "other nations across the Middle East will want to".
Without "the safety mechanisms" of the US-USSR rivalry, Mr Hague said it would be "a disaster in world affairs".
The West suspects Iran wants to develop nuclear weapons, but Tehran insists its programme is for energy purposes.
Mr Hague told the newspaper there was a "crisis coming down the tracks".
"If [the Iranians] 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."
BBC Tehran correspondent James Reynolds said diplomats were concerned about nuclear proliferation.
"If Iran got the bomb then Saudi Arabia would want to get the bomb, then suddenly you would have many countries in the Middle East with nuclear weapons," he said.
Mr Hague's comments come amid heightened tensions in the Middle East, with Israel accusing Iran of masterminding attacks on its embassies in India, Thailand and Georgia. Iran denies the allegations.
It blames Israel and the US for the assassination of several Iranian nuclear scientists in recent years, allegations they deny.
Speaking earlier this month, US President Barack Obama emphasised that Israel and the US were working in "unison" to counter Iran.
However, some commentators have suggested that behind the scenes Washington is deeply alarmed by reports that Israel may strike Iran as early as April. US Defence Secretary Leon Panetta reportedly said there was a strong likelihood of such an offensive.
Mr Hague told the Telegraph that Britain has urged Israel not to strike: "We support a twin-track strategy of sanctions and pressure and negotiations on the other hand.
"All options must remain on the table" but a military attack would have "enormous downsides", he said.
Meanwhile, on Friday, the US and European Union expressed optimism at the possibility of a resumption of talks with Iran.
US Secretary of State Hillary Clinton said a letter from Iran to the US and its allies was "one we have been waiting for".
However, our correspondent said that while Iran had often offered to talk, Western diplomats complained it would steer discussions away from its nuclear programme to leave "parallel monologues" rather than negotiations.
Talks between Iran and six world powers - the US, UK, France, Germany, Russia and China - on Tehran's nuclear programme collapsed a year ago.
In recent months, Western countries have stepped up pressure on Iran over the nuclear issue, with the EU and US both introducing wide-ranging sanctions on the country.
On Wednesday, Iran staged an elaborate ceremony to unveil new developments in its nuclear programme, It said it had used domestically-made nuclear fuel in a reactor for the first time.
There are a number of sites at the centre of concerns over Iran's nuclear programme.