The UK is said to be secretly preparing for a direct military assault by Russia as senior officials warn the country is ill-equipped for a modern conflict on home soil.
The Cabinet Office is reviewing whether to update Britain's 20-year-old 'homeland defence plan' that sets out how the Government responds to a declaration of war, according to The Telegraph.
The move comes after a series of chilling threats from Russia and mounting concerns in Whitehall over the nation's civil and military readiness.
Security officials have warned that Britain would be 'outgunned' by the Kremlin and its allies in a full-scale conflict.
Former Chancellor Philip Hammond recently declared the country is 'massively under-strength' and said more needed to be done to prepare civilians, who he feared were woefully unprepared, for the potential risk of conflict.
Not only would Britain be the weaker side on the battlefield, but it is also vulnerable to an attack on its critical national infrastructure, including gas terminals, undersea cables, nuclear power plants, and transport hubs, experts say.
The updated strategy is expected to detail how the Government would respond to a full-scale attack by a hostile state, including missile strikes, large-scale cyber disruption, and even the use of nuclear weapons.
It will also include contingency plans for safeguarding ministers, evacuating the Royal Family, and coordinating emergency services during a national security crisis.
A senior government source told The Telegraph that the plan would update the dormant War Book to account for the 'new realities of warfare'.
That includes cyber attacks, satellite sabotage, and hypersonic missile strikes that could evade existing defences.
An update to the classified 'homeland defence plan' will set out a strategy for the days immediately after a strike on the UK mainland by a hostile foreign state.
The plan, by the Cabinet Office's Resilience Directorate, will direct the Prime Minister and Cabinet on how to run a wartime government and when they should seek shelter in the Downing Street bunker or outside London.
The war strategies for the rail and road networks, courts, postal system and phone lines are all expected to be reviewed.
A risk assessment published in January warned that a successful attack was 'likely to result in civilian fatalities as well as members of the emergency services,' while also causing serious economic damage and disruption to essential services.
Last month a simulation run in the wake of Vladimir Putin's full-scale invasion of Ukraine showed Britain could not prevent all strikes getting through.
The scenario from 2022 was revealed by Air Commodore Blythe Crawford, the former head of the RAF Air and Space Warfare Centre.
The simulation - part of the RAF's £36million Gladiator programme - looked at how 'day one' of the conflict would unfold.
The UK faced 'hundreds of different types of munitions' attacking from various directions.
Air Commodore Crawford said the outcome was 'not a pretty picture', with some missiles making it through.
He stressed that significant work had been done since then to bolster the defences.
'We [loaded] night one of Ukraine into that synthetic environment and played it out against the UK and, as you can imagine, it was not a pretty picture,' he said.
'It reinforced the fact that we really need to get after this.'
Kremlin officials have repeatedly threatened to attack the UK over its support for Ukraine and last month Putin's propagandists declared British blood 'must be spilled' after they accused Britain of supplying the explosives that killed a top general in a Moscow car bomb.
Lieutenant General Yaroslav Moskalik died in Moscow near his home in the eastern suburb of Balashikha after a Volkswagen Gold filled with explosives was detonated in his presence.
The dead military man was a deputy head of the main operational department in the General Staff of the Russian army.
While the Kremlin previously blamed Ukraine for the attack, Putin's propagandists have now turned their ire on Britain.
So-called military expert Andrei Klintsevich told Russia 1 that Britain's security service handed explosives to the perpetrators 'by the ton.'
Propagandist Vladimir Solovyov angrily added: 'We do realise that someone is creative a network of planted explosives and [transporting] these explosives.
'When we say that British security services are behind every terrorist attack, it means that the blood of the British who authorised the killings on Russian soil must be spilled.
'They must realise that they will pay personally. An eye for an eye, a tooth for a tooth.'
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