Sanctions against North Korea have failed and the threat of military force should not be taken off the table, former Liberal Democrat leader Lord Ashdown has warned.
The peer, who served in the Royal Marines and SBS, disputed ministerial claims the decade-long policy pursued against Pyongyang had worked.
The former UN peace envoy to Bosnia argued there was now a nuclear-armed rogue state with "a world standing on the very brink of nuclear conflagration".
Lord Ashdown made his comments in response to a Government statement on the crisis, sparked by North Korea's latest nuclear test.
International efforts are under way to bring about a diplomatic solution after Pyongyang said it had detonated a hydrogen bomb at the weekend.
Lord Ashdown said:
"I really think it is important at this stage that we should face up to reality."
Rejecting Government claims sanctions had worked, he said:
"This policy followed for the last 10 years has not succeeded, it has failed. What we have got now is a fully nuclear-armed North Korea and a world standing on the very brink of nuclear conflagration. I am not in favour of removing the military option from the table. Our policy so far, however, has been 'respond to our threats and then we will talk'. Why do we not talk while still keeping the threats on the table? Diplomacy works best if it is backed by a military option. There is no reason why we should not now keep that threat on the table and commence some kind of dialogue."
Lord Ashdown went on: "Surely it is better for us to admit that a policy has failed when it is manifestly evident that it has, than to allow the world to come to the brink of this kind of crisis, which can lead to incalculable destruction."
Responding for the Government, Tory frontbencher Baroness Goldie disagreed that sanctions had failed.
"The reaction of North Korea to the sanctions regime has certainly not been positive. North Korea seems to be feeling the effect of the sanctions. That suggests to me that they are indeed having an impact on the economy of North Korea."
She added: "He takes the view that unless a military solution is on the table nothing else is going to work because your discussions don't have any clout.
"But I think we have to acknowledge that a military solution would create a very grave situation.
"It would obviously present big risks to the people of South Korea and it's very difficult to see how that risk could be eliminated.
"I think the UK government is of the view that what we need to pursue is a diplomatic, if possible, economic solution, to try and change the mindset of Kim Jong-un."