The experts' step-by-step guide to surviving a nuclear attack: How to stop your skin melting, why to keep your mouth open so your eardrums don't burst, use soap but not conditioner, and what you REALLY need after the blast...

Midnight: Moscow launches an intercontinental ballistic missile bearing a nuclear warhead at the UK. Twenty minutes to impact.

12.05am: Nato becomes aware of the threat and alerts national governments.

12.10am: News channels and radio stations start broadcasting an emergency warning to the public to seek shelter.

12.13am: Britain's Ministry of Defence restricts citizens' access to 'base transceiver stations', stopping phones from sending messages. This affords the Government, via a secure network, a clear line of communication to every UK-registered device.

12.15am: The Home Office sends a signal to every British mobile phone, warning of the threat.

12.18am: Perhaps half the UK population knows what is coming. The time it takes to boil a kettle, if that, is all most of us have to prepare for a nuclear bomb.

So what should you do with your two minutes? And the 24 hours after that? And the weeks and months of 'nuclear winter' that would likely follow, in which radioactive fallout will have decimated the food chain, contaminated the water supply and caused the total breakdown of civil order?

The first thing to make clear: you can't outrun a nuclear bomb. As newsreader Peter Donaldson warned in his Cold War-era message on behalf of the BBC in the event of a nuclear attack: 'Remember, there is nothing to be gained by trying to get away.'

A nuclear attack on Britain from a hostile state such as Russia is most likely to involve one or more 'atom bombs' with an explosive yield equivalent to one megaton of dynamite. This is 80 times the strength of the Little Boy bomb dropped on Hiroshima, which had a yield equivalent to just 15 kilotons of dynamite and killed up to 140,000 people.

According to modelling by the website NukeMap, a single bomb on this scale would likely kill everyone and destroy all buildings within a 1.4 mile 'heavy blast radius.

As far as seven miles from the impact zone, anyone outdoors can expect third-degree burns, with exposed limbs requiring amputation.

If the bomb were to hit Parliament Square in Central London, for example, immediate fatalities would be recorded as far away as Wembley, Wimbledon and Woolwich, with a northerly breeze taking lethal radioactive fallout as far as Newcastle within an hour.

You have two minutes, then. And you need to find suitable shelter. Your best option is the basement of a large, multi-storey concrete building with as few windows as possible — these will shatter within seconds of impact.

If you have no access to a basement or underground car park, head for the most structurally integral point in a building, such as the central staircase in a block of flats, away from exterior walls.

The greater the density of material you can put between yourself and the blast, the better your chance of survival.

A lead-lined bunker is the ideal — but not possible for most. In the 1980 Government pamphlet Protect And Survive, families were urged to bundle themselves into kitchen cupboards in their homes. This theory remains sound.

Avoid hunkering down in a timber structure such as a shed. This would be like seeking safety in a bonfire. Stay away from long corridors and hallways, too, which will only serve as wind tunnels for the imminent 'pressure wave'.

Keep away from tall furniture such as wardrobes, as these will likely topple. Lie face down, with your arms and hands tucked under your body. The United States Centre for Disease and Control Prevention advises this as the best position to shield exposed skin from the oncoming 'thermal pulse' (a blast of heat energy).

When the bomb hits, the first thing you'll notice from your position on the floor is a dazzling burst of light. In clear weather, this 'atomic flash' can permanently blind someone 10 miles away, burning through their retinas.

At night, when our pupils are dilated, the 'blinding radius' is even greater. Do not look up. Shortly after the first flash, a second one will appear as light previously trapped behind the bomb's 'shockwave' escapes.

Within ten seconds of impact, a fireball 5,700ft across, with a temperature five times hotter than the sun's core (which is 15 million degrees Celsius) will roar across the landscape. As it rises rapidly, it forms into a terrifying mushroom cloud. Then you can expect the pressure wave — an unstoppable battering ram of highly compressed air travelling at 784mph, faster than the speed of sound. Irwin Redlener, an American expert in public health and disaster response, advises keeping your mouth open 'so your eardrums don't burst' as a result of the sudden increase in pressure.

The thermal pulse will ignite any flammable materials within several miles of Ground Zero, including dry leaves, curtains, newspapers and clothes.

Wearing pale colours will reduce the risk of your own clothes catching fire, as they will reflect rather than absorb this heat.

Within ten to 12 minutes, lethal radioactive debris known as 'fallout' will begin to rain down from the sky like a sandstorm.

It's time to make a decision: stay put and trust in the integrity of your shelter, or make a break for it, hoping to escape the fallout zone — which could stretch as far as 250 miles from the blast.

In all but the most extreme circumstances, it's best, as the Government recommends in its current official slogan, to: 'Go in. Stay in. Tune in.'

If you have an electric car, this will be useless due to the electromagnetic pulses (EMP) — sharp bursts of electricity — emitted by the bomb, which have caused voltage surges and destroyed components. In any case, few roads will be passable and cars for several miles from Ground Zero are just scraps of charred metal.

But if you do decide to flee, travel upwind to avoid being chased by the fallout cloud.

If your upwind course takes you towards Ground Zero, travel cross-wind instead.

High doses of radiation melt the body from the inside. Vomiting begins within 30 minutes of exposure and is followed by diarrhoea and loss of consciousness. Death can take just hours.

At lower doses, symptoms of radiation poisoning include severe weight loss, internal bleeding, hair loss, hypotension (low blood pressure) and total loss of bowel control. Terminal cancers such as leukaemia may develop.

You must therefore try to prevent radioactive material from entering your lungs.

Cover your nose and mouth with a cloth or ideally a P2-grade face mask. Breathe through your nose: mucus-covered nasal hairs are capable of filtering 95 per cent of foreign particles exceeding five micrometers in diameter (0.005mm). Wear glasses or goggles. Stick cotton wool in your ears and cover any open wounds.

If you remain in your shelter, try to ensure it is as secure as possible. Seal off any units — such as air conditioning and fireplaces — that let in air from the outside. This is your home now. Place mattresses against shattered windows and cover door frame gaps with gaffer tape.

Do not go looking for loved ones or pets. Buildings have been destroyed, corpses litter the roads. Going outside will inevitably end in panic, disorientation and death.

Anyone who hopes to join you in your shelter must decontaminate. Nobody can touch them or their clothes until they've washed, ideally with warm water and soap.

Check their body for third-degree burns: the nerves beneath their skin may have been destroyed, rendering the wounds painless. Shampoo should be used to clean hair, but under no circumstances use conditioner. According to the U.S. Federal Emergency Management Agency, conditioner contains compounds known as 'cationic surfactants' that trap and bind radioactive particles to the scalp. For the same reason, do not use any body lotions, creams or moisturisers.

Use a damp cloth to wipe inside your ears and across your eyelids, where radioactive dust can settle. Scrub under your fingernails, making sure not to break or damage the skin. Gently blow your nose. Place any used towels, cloths and clothes into a bin bag: seal this and place it out of reach.

Burns should be covered with a dry dressing and left unchanged for at least a week. Remember to breathe: adrenaline is your friend, but panic at this time could be fatal. The Prime Minister is relatively safe in a network of nuclear tunnels beneath Whitehall. As he prepares to mount Britain's response — launching a nuclear attack on Moscow, perhaps — civil disorder is a risk.

You may want to arm yourself with knives, machetes and bludgeons for protection if you venture out — and a gun if you have one. Do not rely on the police or army to maintain order: the only currency from now on is likely to be brute strength.

Although the Whitehall bunker has an access hatch within Buckingham Palace, the King has been evacuated to a separate bunker beneath the state rooms.

Power lines, phone masts and internet cables will likely be useless. If you have access to a battery-powered radio, tune it to any frequency with signal.

Survivalist Cresson H. Kearny's 1986 guide to surviving a nuclear war urged people to 'listen only periodically' and preserve battery life by ensuring the radio is played 'at reduced volume'.

If you do get a signal, it is likely to be coming from the BBC's Wood Norton nuclear bunker in Worcestershire, which has access to a super-high frequency satellite station on a nearby hill.

Although the Beeb claims the site is now only used for 'technical training,' one insider has said that the bunker remains the Corporation's 'point of final stand'. A healthy person can survive without food for up to three weeks provided they drink about three pints of water per day.

Begin by carefully rationing any reserves of bottled water available to you. Many buildings and homes have dedicated water tanks: these will be safe to drink from. So is water held in the flush mechanism of a lavatory, but do not drink directly from the bowl.

You might also find water in tinned foods such as beans and soft fruits.

The Government advises drinking mains water only as a last resort — it is likely to be contaminated. Please note: water that contains radioactive particles is not made safe by boiling. However, it can be safely filtered through clay soil, which you might find in some indoor pot plants.

If you possibly can, take iodine tablets to protect against thyroid cancer caused by radiation exposure. As Dr David Brownstein, a specialist in thyroid conditions, explains: 'If there is enough inorganic, non-radioactive iodine in our bodies, the radioactive fallout has nowhere to bind. It will pass through us, leaving our bodies unharmed.'

If you can't source supplements, iodine-rich foods include seaweed and potato peel.

As the days drag on, your mind can become as much of a prison as your shelter. If you have a battery-powered light such as a torch, use it sparingly to aid your circadian rhythms. Try to stay awake during the day and sleep through the night to avoid becoming disorientated.

Two weeks after impact, the radioactive fallout will have declined to just one per cent of its original strength. It is now safe to come out.

But what sort of a world will you find? The worst-case scenario is that global nuclear war has created a dense sheet of fog, formed of dust and smoke, which has settled so high in the atmosphere it does not come down.

This sheet prevents sunlight from reaching earth. Temperatures in Britain will drop to as low as -25c, even in July.

This is the 'nuclear winter', a theory established by academics — including the late Carl Sagan — in a 1983 paper, in which they concluded: 'The possibility of the extinction of Homo Sapiens cannot be excluded.'

With crops destroyed, citizens will turn to the Millennium Seed Bank in Sussex, which preserves 2.4 billion seeds in deep underground caverns.

With no sunlight and limited electricity, crops that grow in shade will be prioritised, such as spinach and chard, as well as root vegetables such as carrots, leeks and parsnips.

In the short term, with contamination rendering most farmland barren, crops that can be grown without soil, such as micro-greens, will be important.

Any animals with radiation sickness will die and should not be consumed.

Any animal that appears healthy can be slaughtered for food — but do not eat the thyroid, kidneys or liver, as they will certainly be contaminated.

A study published two years ago in the Nature Food academic journal estimates that in the event of a nuclear winter, five billion of the world's eight billion population would die from famine.

The first few weeks, therefore, are only the beginning. A nuclear winter will likely see the breakdown of civilisation as we know it. Seeds will be worth more than gold. Sunlight will be a memory.

But if our species does survive, then eventually — years down the line — the deadly smog will lift, the light will break through and humanity will have a chance to begin again.