Donald Trump has an "absolute duty" to prepare for military intervention against North Korea in the face of the mounting nuclear threat posed by the regime, Boris Johnson will say.
The Foreign Secretary will warn that Kim Jong-un, the North Korean leader, is on the verge of acquiring that capability of "make good his threat" to reduce New York to "ashes".
It comes after the US President yesterday said that the US is "totally prepared" for war with North Korea as he increased the US military presence in North Korea. He has previously threatened to unleash "fire and fury".
Mr Johnson, speaking at the annual Chatham House conference in London, will say: "I am afraid that the US President – whoever he or she might be – will have an absolute duty to prepare any option to keep safe not only the American people but all those who have sheltered under the American nuclear umbrella."
However he will urge Mr Trump to show "far-sightedness" and not quit the Iran nuclear deal, arguing it shows the kind of "diplomatic imagination" that could end the North Korea crisis.
Mr Johnson will single out US secretary of state Rex Tillerson for praise for opening the door to dialogue with Kim Jong-un, which Mr Trump has described as a "waste of time".
He will say that diplomacy remains the best way to avoid a nuclear war and highlight the importance of the 1970 Nuclear Non-Proliferation Treaty in removing the threat of a "nuclear winter".
He will say: “A new generation has grown up with no memory of the threat of a nuclear winter, and little education in the appalling logic of mutually assured destruction. The memory of Hiroshima and Nagasaki is now literally fading from living memory.
“The NPT is one of the great diplomatic achievements of the last century. It has stood the test of time. It is the job of our generation to preserve that agreement, and British diplomacy will be at the forefront of the endeavour.”
The Foreign Secretary will restate the "astonishing" and "extraordinary" achievement of the treaty and point out its success rests on nations choosing to "take shelter under a nuclear umbrella" provided by the US.
The "American offer" and guarantee is one of the "greatest contributions" by the country to the "unprecedented epoch of peace and prosperity that we have all been living through", he will say.
North Korea | Kim Jong-un’s fiery relationship with the US
Mr Johnson will argue that while other military developments, from firearms to fighter jets, have "spread among humanity like impetigo" nuclear weapons remain "the great exception" because of the agreement.
"It has helped avoid what might otherwise have been a Gadarene Rush to destruction, in which the world was turned into a great arena of Mexican stand-offs, a nuclear version of the final scene of Reservoir Dogs.
"That far-sightedness is now needed more than ever, not only to keep the NPT, but also one of its most valuable complementary accords, the nuclear deal with Iran.”
Stressing the need to keep military options on the table, he will say: "That is the model - of toughness but engagement, each reinforcing the other - that we should have at the front of our mind as we try to resolve the tensions in the Korean peninsula.
"It is right that Rex Tillerson has specifically opened the door to dialogue. He has tried to give some sensible reassurances to the regime, to enable them to take up this offer."