Iran's nuclear ambitions could plunge the Middle East into "a new Cold War", the UK foreign secretary has warned.
William Hague told the Daily Telegraph other nations in the region would want to develop nuclear weapons if Iran did.
Without "the safety mechanisms" of the US-USSR rivalry, Mr Hague said it would be "a disaster in world affairs".
But ex-UK diplomat Sir Richard Dalton said Iran was not "rushing towards a nuclear weapon". Tehran insists its programme is for energy purposes.
The West suspects Iran wants to develop nuclear weapons.
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."
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.
Shashank Joshi, of defence think tank the Royal United Services Institute, told the BBC the West's fears could be unfounded.
"If we could live with nuclear weapons in the hands of totalitarian, genocidal states like Stalin's Russia or Mao's China, Iran in contrast - whatever its repulsive internal policies and adventurism abroad - is far more rational," he said.
Mr Joshi said Iran may not be actively pursuing the creation of nuclear weapons but leaving the option open.
"If they feel their regime is under existential threat, if they feel they face a Libya-like situation, they would have the option of building a bomb."
Sir Richard, a former UK ambassador to Iran, said: "There are many signs, as reported by the International Atomic Energy Agency, that some research and development relevant to the development of nuclear weapons may still be going on.
"But it is wrong to say that Iran is rushing towards having a nuclear weapon.
"Indeed, the analysis published to the United States Congress by the top intelligence assessors there indicates that Iran has not taken a decision to have a nuclear weapon.
"But it is right that the IAEA should press Iran on behalf of the international community to answer fully questions about what it has been up to in the past and what it may still be doing in the present."
Shadow foreign secretary Douglas Alexander said: "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."
Meanwhile, Iranian warships have entered the Mediterranean Sea for only the second time since the 1979 revolution.
The destroyer Shahid Qandi and its supply vessel Kharg have passed through the Suez Canal but their destination remains unclear.
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.