A high-pitched siren rings out for 10 seconds from 85 million mobile phones. The emergency alert warns members of the public to “go in, stay in, tune in”, and keep away from external walls and windows to avoid radiation.
And at the heart of power, a well-rehearsed plan that no one ever wants to see put into action clicks remorselessly into place.
These are some of the opening moments of what is expected to happen if a nuclear strike on the British mainland is imminent.
While this scenario remains unlikely – given the UK’s nuclear deterrent and Nato’s early warning and response plans – it has been brought back into sharp focus following the publication of the government’s Strategic Defence Review.
That review made clear the threats to the UK from adversaries like Russia – and potentially, China – are becoming graver.
The SDR warned that Moscow and Beijing are “putting nuclear weapons at the centre of their security strategies, increasing the number and types of weapons in their stockpiles”.
Defence insiders connected to the SDR believe there is no need for the public to “panic” about the prospect of a Third World War.
But the review called on the government, industry and society to take steps to bolster the UK’s homeland defences.
A Government spokesperson said: “We have robust plans in place for a range of potential emergencies that have been developed and tested over many years.”
This includes the Home Defence Programme, a team that helps coordinate the government’s planned response to potential hostilities affecting the UK.
Russia is an ‘immediate and pressing threat’
The SDR warned that states such as Russia are “intentionally blurring the lines between nuclear, conventional, and sub-threshold threats, complicating the ability of the UK and its allies to manage potential escalation and miscalculation”.
When it comes to Nato members, Russia has focused its hostilities on so-called “grey zone” or “sub-threshold” activity – such as cyber attacks and interference with under-sea communications cables in the Baltic and North Sea, as well as amassing troops and equipment on its border with Finland, a new Nato ally.
Describing Russia as an “immediate and pressing threat”, the SDR warned that despite the focus on grey zone activity, Moscow is also building up its nuclear stockpiles, as is Beijing.
The SDR said: “The coming decades will be defined by multiple and concurrent dilemmas, proliferating and disruptive technologies, and the erosion of international agreements and organisations that have previously helped to prevent conflict between nuclear powers.”
Keir Starmer’s government has been at pains to differentiate between Russia, the aggressor that invaded Ukraine, and China, a country that presents a strategic challenge yet still offers opportunities for cooperation.
But the SDR pointed out that Beijing has instigated a “vast increase in advanced platforms and weapons systems, such as space warfare capabilities” as well as the “unprecedented diversification and growth of its conventional and nuclear missile forces, with missiles that can reach the UK and Europe”.
China has “more types and greater numbers of nuclear weapons than ever before, with its arsenal expected to double to 1,000 nuclear warheads by 2030”.
While Vladimir Putin will be well aware of the consequences for Russia if he authorises the use of a nuclear weapon against a Nato country, triggering Article 5 – which states that an attack on one member invokes a response by all – there has always been the risk of an accidental or miscalculated strike.
This week, Russia’s ambassador to the UK Andrey Kelin ramped up pressure on Starmer’s government by claiming it had helped Ukraine carry out deadly drone attacks on his country.
He told The World with Yalda Hakim on Sky News that Kyiv should “not try to engulf World War Three”.
But Kelin did insist that “we do not represent any threat for Britain at all, neither on the sea, nor in the air, nor on the earth”.
Fully equipped bunker under Whitehall
If a nuclear attack is imminent – allowing just minutes to take cover – Britain’s seat of power would swiftly move 200ft underground.
Since 1992, a reinforced, high security bunker beneath Whitehall – deeper than most Tube lines – has been standing by as the refuge of last resort in the event of an incoming nuclear strike on London.
Known as “Pindar” after the Ancient Greek poet whose home was spared by Alexander the Great, it has undergone an upgrade in recent years to modernise its situation room and living quarters.
There are understood to be 400 bunks, and access passes for designated ministers, officials, special advisers and the military are ready by the entrance inside the Ministry of Defence, regularly updated to take account of changes at the top.
Starmer, his top officials and Cabinet ministers, along with their advisers, would gain access to Pindar from Downing Street via a tunnel under Whitehall.
A RS-24 Yars intercontinental ballistic missile system drives across Red Square during the Victory Day military parade in central Moscow on May 9, 2025. Russia celebrates the 80th anniversary of the Soviet Union’s victory over Nazi Germany in World War Two (WWII).
An RS-24 Yars intercontinental ballistic missile system drives across Red Square during the Victory Day military parade in central Moscow on 9 May.
From here, the Prime Minister would authorise a military response – including the deployment of a Trident nuclear missile from one of the UK’s submarines against the aggressor.
Inside the situation centre, the PM and his officials would also oversee domestic operations – this would likely include instructions to the police and the army to prevent looting and public disorder in areas outside the blast site and in other towns and cities.
PM evacuated to ‘AHQ’ in High Wycombe
If there is more warning of an attack – say, for example, the UK is already engaged in conflict with Russia, and therefore at a higher state of readiness for a potential strike on the capital – it is more likely that the PM, top military and ministers would be stationed at an alternative headquarters at RAF High Wycombe in Buckinghamshire.
This has day-to-day use as the MoD’s air command as well as space command – monitoring threats in the skies as well as, potentially, exoatmospheric intercontinental missiles launched by an enemy state.
As alternative headquarters or “AHQ” it could accommodate the PM and Cabinet, as well as ongoing military operations, to ensure the ongoing running of government.
High Wycombe also has the advantage of being close to Chequers, the Prime Minister’s weekend residence.
The AHQ would be linked to the government’s recently installed National Situation Centre in the Cabinet Office in Whitehall, if London was still unaffected.
High Wycombe could also be used in the event of a conventional missile attack on London. In this scenario, the capital would be damaged by missiles in the way that Kyiv has sustained strikes by Russian weapons, but it would still be operational.
It is also understood that some ministers, officials and military would be dispersed to “hubs” in different regions in case the main centre of power, where the Prime Minister is located, was struck.
This would also apply to the Royal Family to protect the line of succession –- the King and Queen and Prince and Princess of Wales, along with their children, would be evacuated to different remote and secret locations outside of London.
Current command HQ carries ‘unnecessary risks’
The SDR raised concerns about potential vulnerabilities of Permanent Joint Headquarters, the current main centre of military operations for the armed forces, based at Northwood.
Its location – in a suburb of north London – may make it susceptible in the event of a physical attack by Russia, while there is also a potential cyber threat, the review said.
The SDR warned this “poses unnecessary risks to the assured command of future UK military operations, including in defence of the UK itself” and that “the vital role of Permanent Joint Headquarters in commanding UK military operations means the MoD should ensure it is more resilient to both physical and cyber-attack”.
The project to upgrade and refuel the nuclear submarine will safeguard 2,000 UK jobs and will take around three and a half years. River Tamar shipping was suspended when the the 150m-long, 15,900-tonne submarine HMS Vengeance travelled to the Devonport Dockyard ahead of the scheduled refit on March 2, 2012, arriving amid concerns without a signed contract for the refit. HMS Vengeance will be the last of four Vanguard class submarines to undergo a Lond Overhaul Period and Refuel LOP(R) at Davernport that will commence once HMS Vigilant leaves Devonport on completion of her LOP(R) in 2012. Mandatory Credit: (Photo by Andrew Linnett/MoD Crown Copyright via Getty Images)
HMS Vengeance – one of four submarines carrying Britain’s nuclear deterrent (Photo: Andrew Linnett/MoD Crown Copyright via Getty Images)
Defence insiders said PJHQ plays a vital role in UK military operations and that it was resilient to both physical and cyber attacks, but that this resilience would be reviewed periodically as technology and warfare changed.
The MoD will develop plans and determine any required investment in the future following the SDR recommendation, insiders said.
Field hospitals and ‘buddy cities’
While the government would muster a response and try to keep order, what about the rest of us?
That emergency alert pinged to our phones – urging us to go inside and stay there, while listening for further updates from authorities – may be useful away from the blast site, but inside and near to the zone it would be, of course, a mass casualty event.
One of the recommendations of the SDR was for the NHS to boost stockpiles of medicines and equipment to prepare for a nuclear attack.
According to World Health Organization guidance, this would likely include medicines such as KI or potassium iodide tablets to reduce the risks from radiation exposure, drugs to prevent diarrhoea and nausea, and antibiotics.
Field hospitals outside the blast zone would also house personal protective equipment, trauma kits for possible burns, fluids and pain medication, CBRN [chemical, biological and radiological] suits, gas masks and air sampling kits, decontamination equipment and portable field showers.
Contingency plans for regional cities away from the direct blast zone would also be implemented. Every council and public sector organisation has a contingency plan for various threats, rising to Level 3 or “catastrophic” emergency.
But a recent report by the National Institute for Health and Care Research also recommended the creation of “buddy cities”, which would come to the aid of a city struck by a nuclear bomb for those not in the direct blast zone and killed immediately in the attack.
Each UK city would have two “buddies”, which would house equipment and medical supplies, protective vehicles and clothing for police and evacuation services, as well as burns centres, designated food supplies for displaced residents, and housing for evacuees.
The report also recommended the use of loudspeakers in affected areas for authorities to convey urgent information because mobile phone signals would not work in the wake of a nuclear attack.
Plans are top secret – but here’s how we can prepare
Detailed plans for what the UK government plans to do in the wake of a nuclear attack are not publicly available, however.
The latest edition of the government’s National Risk Register, published in January, lists a conventional attack on the UK mainland, as well as a nuclear “miscalculation” not involving the UK.
But the risk register makes clear that the full plans for a nuclear bomb used on Britain are top secret. In a footnote it says: “A separate scenario involving a nuclear attack on the UK mainland or UK overseas interests exists and is held at a higher classification.”
The NIHR report criticised the government for failing to make public its plans for dealing with a nuclear strike, saying: “There will be little that the government can do if the country is not prepared.”
This may be because sharing plans would give the UK’s enemies an advantage, although the report said this argument was “not convincing” because other countries make their plans available for the public to prepare.
Since the end of the Cold War, many local nuclear bunkers have fallen into disuse or disrepair, meaning the public may instead need to use underground transport systems to shelter, if they can get there quickly enough.
The government does however publicise advice for what the public should do in a “radiation emergency” – which is to: “Go in, stay in, tune in”.
In fact, this message is similar to the one the UK government used to prepare for a nuclear attack, last issued in 1980: “Get inside, stay inside, and stay tuned.”
The updated advice for a radiation emergency says people should “go indoors as soon as you can and stay there. Close the doors and windows and stay as far away from external walls as possible”.
It adds: “Staying indoors reduces your exposure to any radiation outside. You will be given advice on what to do next.
“Turn off all fans and air conditioning, and close all external doors, windows and vents where possible. You may need to stay indoors for one to two days.”
And the public should “tune in” to official alerts on TV, radio, government websites or social media.
But there are things people can do now to get ready – just in case the worst does happen.
The government has a Prepare website for the public to equip their homes for possible emergencies, from natural disasters to armed conflicts.
Listed is a 72-hour survival kit it recommends every household should have. Items include bottled water, long-life food, wind-up radios and torches, and first aid equipment.