As tensions reach boiling point between the US and hermit kingdom, a defence think thank has warned hundreds of thousands of people are likely to be killed or injured.
In a disturbing assessment of the crisis sparked by Pyongyang's intensified nuclear missile testing, the Royal United Services Institute (Rusi) said combat would "not be surgical or short".
World War 3 could be triggered by either North Korea or the US but there is a growing risk that Donald Trump will decide to "resolve" the issue "sooner rather than later", according to its report.
The UK would have only a few hours at most to decide how to respond, the chilling report warned.
The report said: "War is now a real possibility.
"With North Korea making rapid progress in its missile and nuclear programmes, time is not on diplomacy's side."
Pyongyang accused the US of declaring war after Mr Trump tweeted that crackpot despot Kim Jong-un "won't be around much longer".
The White House dismissed the claim but Mr Trump has insisted that if the US was forced to defend itself against the rogue state "we will have no choice but to totally destroy" the country.
But in the Rusi report, written by its deputy director general, Malcolm Chalmers, there is a number of ways a war could be started are set out, such as a pre-emptive strike by Pyongyang if it believed the US was planning a surprise attack.
Washington could be provoked into an assault if North Korean test missiles were fired into the ocean near California or Guam, an American territory, it said.
An outbreak of war would be likely to involve an early large-scale US-led air and cyber offensive, followed by retaliation by Pyongyang against South Korea - where around 8,000 Britons live - and American bases in the region using conventional, chemical or possibly nuclear weapons.
That would mean a full-scale invasion of the state would be highly likely, the report said.
Even without the use of nuclear weapons, the casualties would probably reach the hundreds of thousands and the war would also have "far-reaching consequences" for the global economy.
A US preventive strike without South Korea's agreement would send a signal that Washington was willing to "sacrifice Seoul to protect New York".
The report calls on the UK government to continue work to intensify international sanctions on North Korea and to support efforts to find a diplomatic solution.
It must also urge the US to rule out preventive strikes and put preparations in place to deal with a range of military scenarios, the report says
But the UK government should refuse to rush into unconditional support for US action if it launches a surprise attack and must listen to South Korea and Japan, neighbouring nations that would be hit hardest by the fallout.
The report states: "The UK government would have, at most, only a few hours to make clear how it stood on what would be one of the most momentous strategic shocks of the post-Cold War era. Its decision would have as profound an impact on the UK's international standing, and on its domestic politics, as the fateful decision to stand 'shoulder to shoulder' with the US in the run up to the 2003 invasion of Iraq."