Former Foreign Secretary Lord Hurd has said Britain cannot "realistically" rule out using military force against Iran over its nuclear programme.
Lord Hurd said any action would involve 'huge dangers'
But any such action would involve "huge dangers" and governments must show "patient strength", he told peers.
On Tuesday, Tony Blair said using force against Iran was "not on our agenda".
Iran has said it will no longer allow snap inspections of its nuclear sites, after the UN nuclear atomic voted to report it to the Security Council.
On Tuesday, Mr Blair told the Commons liaison committee Tehran was making "a very, very serious mistake" if it thought the international community would allow it to develop nuclear weapons.
But he said the issue could be dealt with by "peaceful and diplomatic means".
Opening a House of Lords debate, Lord Hurd said: "I don't think we can realistically and forever rule out the use of force.
"If the regime in Iran or its successor moved from words and piled up an unmistakeable danger then I don't think we can entirely rule out the use of force.
"But we shouldn't deceive ourselves than we can have some sort of strike without a war, or some sort of war that doesn't involve huge dangers and many, many thousands of casualties."
Lord Hurd added: "We have chosen to station our troops, whether in southern Iraq or southern Afghanistan, in very modest numbers where we are uniquely vulnerable to retaliation from nearby Iran."
He later told the BBc there was no such thing as a "surgical strike against a nuclear installation" because the country attacked would be likely to retaliate.
Iran, a "proud and ancient nation", had to be offered incentives to comply with international wishes, Lord Hurd added.