Britain is reportedly preparing for the possibility of war breaking out with North Korea as concerns rise that another provocative missile test could trigger a military response by the US.

North Korea is being closely watched amid fears it could launch another long-range missile test on Tuesday to mark the anniversary of the founding of its ruling party.

Bellicose rhetoric from Donald Trump has heightened tensions in the region in recent months, prompting British officials to draw up military plans for a response to a break out of hostilities, it was reported.

Among the plans disclosed by the Daily Mail is the deployment of the Navy's newest aircraft carrier, HMS Queen Elizabeth, before it has undergone flight trials.

"We have plenty of ships to send… the Type-45 destroyers, the Type-23 frigates. Britain’s new aircraft carrier could be pressed into service early if things turn south," a senior Whitehall source told the newspaper.