Allowing Iran to build a nuclear bomb would spark a Middle East arms race, Foreign Secretary William Hague has warned MPs.
Mr Hague told the Commons on 20 February 2012 that if the Tehran regime managed to construct a viable weapon, its neighbours would be forced to build their own nuclear warheads too.
He accused Iranian President Mahmoud Ahmadinejad of pursuing "confrontational policies" and described the country's enrichment of uranium in defiance of United Nations Security Council resolutions as "a crisis coming steadily down the track".
The foreign secretary was speaking in a backbench-led debate on a motion, tabled by Conservative MP John Baron, which called on the government to rule out using military force against Iran.
Opening the debate, Mr Baron called on the West to stop its "sabre-rattling" policies over Iran's nuclear ambitions.
The Conservative MP, who opposed both the Iraq war and the military intervention in Libya, claimed that military action against the regime would be "counter-productive".
But Mr Hague, responding for the government, said ministers needed to retain all their options as they tried to pressure Tehran into abandoning its nuclear programme.
The foreign secretary told MPs that the government opposed Mr Baron's motion because it would boost Iran's confidence if the UK were to rule out military action.
"Our policy is that whilst we remain unswervingly committed to diplomacy, it is important to emphasise to Iran that all options are on the table," Mr Hague told MPs.
Tensions over Tehran's alleged attempts to develop nuclear weapons have led to Iran halting oil exports to the UK and France in retaliation for European Union sanctions.
The regime has also threatened to close the strategically important Straits of Hormuz.
UK 'should not blink first'
Sir Malcolm Rifkind, speaking to a cross-party amendment which called for "all options" - including the use of force - to be kept open, warned that Israel would be more likely to attack Iran if the UK were to rule out military action.
The former Conservative foreign secretary said force would "never be a desirable response but it may be the least bad response if all else fails".
Labour's former foreign secretary Jack Straw agreed, telling MPs: "I do not think it reasonable to ask the British foreign secretary to negotiate on this very difficult issue and to remove one possible option that may in distant circumstances be necessary."
Shadow foreign secretary Douglas Alexander agreed that it would be the "wrong course" for military action to be ruled out, but warned against armed intervention becoming a "self-fulfilling prophecy".
"Now is not the time for a pre-emptive strike on Iran. Now is the time for re-doubling our diplomatic efforts to capitalise on the progress that is being made," Mr Alexander said.
Meanwhile, the Foreign Affairs Committee chair Richard Ottaway told the Commons: "It is critical we do not blink first."
He said that diplomacy had failed in Iran and that military intervention was the "least bad option".
Liberal Democrat Martin Horwood disagreed, warning that a preventative attack on Iran "would probably be illegal" and could lead to the "ignition of a war over which we have absolutely no control".
You can watch the concluding part of the debate