The British public must "actively prepare" for war on UK soil, a chilling Government document warns.
The newly-published National Security Strategy said the years ahead will test the nation - with a World War Two-style spirit needed to deal with growing threats.
It said tackling the danger of nuclear weapons will be "more complex than it was even in the Cold War". The document went on to warn that major powers like China and Russia are seeking to gain an upper hand in "outer space, cyberspace, the deep sea, and at the Arctic and Antarctic poles".
And the report also warned that hostile states may join forces with terror groups and criminals to carry out attacks on these shores. It states: "For the first time in many years, we have to actively prepare for the possibility of the UK homeland coming under direct threat, potentially in a wartime scenario."
Warning of hostile states acting within the UK, Keir Starmer said the country is in a period of "radical uncertainty". He has pledged to spend 5% of GDP on national security within a decade - a move the strategy says will draw together civilian and military priorities "in a way not seen since 1945".
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The document says: "The years ahead will test the United Kingdom... The direction it takes – and the decisions we take – will reverberate through the decades.
"We will need agility and courage to succeed, but we should be optimistic. We remain a resolute country, rich in history, values and in our capabilities. But most of all, there is the determination of the British people themselves. After all, we do not need to look too far into our history for an example of a whole-of-society effort, motivated by a collective will to keep each other safe.
"We can mobilise that spirit again and use it both for our national security and the rebuilding of our country."
The strategy said arms control measures had failed to keep up with the spread of nuclear weapons - highlighted by alarm over Iran's nuclear weapons programme. It said: "The threat to the UK and our allies from nuclear weapons is once again growing.
"Tackling this challenge is likely to be more complex than it was even in the Cold War, with more states with more nuclear weapons, the further proliferation of nuclear and disruptive technology, and the failure of international arms control arrangements to keep pace."
And the grim document added that the UK is "directly threatened" by hostile activities including assassination, intimidation, espionage, sabotage, cyber attacks and other forms of democratic interference.
It stated that critical national infrastructure such as undersea cables, energy pipelines, transportation and logistics hubs are also a target. And the strategy warns: "It may become more difficult to identify hostile state activity as they make use of terrorist and criminal groups as their proxies."
And referencing North Korea's support for Russian forces in Ukraine, the strategy said hostile nations are more willing to work together. "As our adversaries and competitors engage in further military modernisation, issueslike technology transfer and sanctions enforcement are going to become increasinglyimportant," the strategy states.
Addressing MPs in the Commons, cabinet office minister Pat McFadden said the Government needs to be "clear-eyed and hard-edged". Mr McFadden said ministers would work on protecting security at home by bolstering borders and making the UK "more resilient to future threats".
He said more would need to be done with international allies to defend collective security - especially in the UK's relations with major powers like China. The strategy warned that "instances of China's espionage, interference in our democracy and the undermining of our economic security have increased in recent years".
Mr McFadden also set out the UK's priority to rebuild its defence industries and building "advantages in new frontier technologies" like AI. "All of this will make us a stronger and more resilient country," he added.
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