A “new Korean War is now a real possibility,” partly because Donald Trump “is so volatile and impulsive”, a leading think tank has warned the British government.

Civil servants were urged by Malcolm Chalmers, Deputy Director General at the Royal United Services Institute, to begin preparations immediately for the possible outbreak of war between the US and North Korea.

Tensions between the two countries have been steadily increasing in recent months, with Mr Trump most recently saying the US was “locked and loaded” in the face of North Korean missile threats against Guam.

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As a close ally of the US, the UK would most likely be asked to provide assistance in the event of a conflict on the Korean peninsula, where the US has around 35,000 troops stationed.

Mr Chalmers’ warning comes as many experienced civil servants have been seconded to Brexit duties, leaving a dearth of knowledge in other departments.

It is feared this could weaken the government’s diplomatic response to a major crisis.

A “dangerous mix” is being created on the Korean peninsula by the North’s technical advances – an intercontinental ballistic missile (ICBM) was recently fired by the country which experts said could reach almost any city in the US – and Mr Trump’s “volatile and impulsive personality,” Mr Chalmers wrote.

“If war did begin, the President would be on the phone to 10 Downing Street within an hour asking for support," he added. “The British prime minister would then have only a few hours, at most, to make clear how she stood on what would be one of the most momentous strategic shocks of the post-Cold War era.”

Despite the enormity of the decision, “there would be little time for multiple consultations and deliberations before positions had to be taken,” Mr Chalmers said.

“The die would be cast on Day One,” he added. “Whitehall needs to be preparing now for a range of possible scenarios that could unfold in the coming months.”

The response offered by the UK would likely depend on who initiated the conflict, Mr Chalmers said. It is believed the UK could have little choice but to offer “strong political support” if King-Jong Un, the North Korean leader, obviously started the conflict.

But the response could be different if the US was seen as having started the war. Mr Chalmers suggested the best support the UK could offer in this scenario was diplomatic, with negotiations aimed at limiting the spread of conflict and ensure both the US and China were happy with any peace agreement.

In pictures: North Korea military drill Show all 8 1 /8 In pictures: North Korea military drill In pictures: North Korea military drill North Korea's leader Kim Jong Un watches a military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video

It was earlier reported that Kim Jong Un delayed a decision on firing missiles towards Guam while he waited for the US’s next move. His plan to fire rockets towards the island, where the US has a key military base, prompted a surge in tensions and a threat from Mr Trump that the American military was "locked and loaded" if it was provoked.

But experts have urged caution on the apparent decline in tensions.