What is the UK's nuclear deterrent?

It's been a week in which Vladimir Putin put Russia's nuclear forces on "special" alert following the West's so-called "unfriendly" response to the Ukraine crisis.

On Sunday, Putin was televised telling two of his military ministers to put Russia's nuclear weapons on a “special regime of combat duty”.

While experts suggested this is rhetoric, and that a nuclear attack remains unlikely, it marked a significant ramping up of Russia's aggression towards Nato, the military alliance currently made up of 28 European countries, plus the US and Canada.

So while the chances of a nuclear attack remain remote, Putin's aggression does at least raise the question: what is the UK's nuclear deterrent? Here, Yahoo News UK explains all.

How many nuclear warheads does the UK have?

According to the Federation of American Scientists, the UK has a stockpile of approximately 225 nuclear warheads.

The UK is one of nine countries which has nuclear warheads.

Watch: Defence secretary labels Putin nuclear talk a 'distraction' (from Tuesday)

The eight others are the US (5,428), Russia (4,477 - which excludes 1,500 "retired" warheads), China (350), France (290), Pakistan (165), India (160), Israel (90) and North Korea (20).

Between the Nato allies of the US, France and the UK, there are 5,943 nuclear warheads.

What is nuclear deterrence?

The UK government says the purpose of stockpiling nuclear warheads "is to preserve peace, prevent coercion and deter aggression".

Its guide to nuclear deterrence states: "Potential aggressors know that the costs of attacking the UK, or our Nato allies, could far outweigh any benefit they could hope to achieve.

"This deters states from using their nuclear weapons against us or carrying out the most extreme threats to our national security."

The government says its nuclear deterrent "protects us every hour of every day" because the UK's option of a "credible and effective response option to extreme aggression" reduces the likelihood of an attack happening.

What is Trident?

The UK government says it has a "credible" nuclear deterrent, kept to a "minimal" amount of destructive power, through its Trident programme.

This deterrent is employed through its "continuous at sea deterrent" (CASD), which has been maintained by the Royal Navy since 1969.

The CASD means there is "at least one [out of a total four] nuclear-armed ballistic missile submarine patrolling the seas undetected at all times, ready to respond to the most extreme threats to the UK".

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As prime minister, Boris Johnson is the only person in the country who can authorise the UK's use of a nuclear weapon.

The four Vanguard-class submarines carry the UK's Trident missiles and warheads, with any decision to launch them sent through to the boat in an encrypted message.

Identical handwritten "letters of last resort", authored by Johnson, are located on each of the four submarines inside a safe system, and are addressed to the commanding officer of each boat.

What about nuclear disarmament?

The UK says it "remains committed to the ultimate goal of a world without nuclear weapons".

But this is hugely unlikely. It would rely on dictators like Putin, Kim Jong-un and Xi Jinping abandoning their countries' nuclear programmes: an improbable prospect.

The UK says: "Abandoning our deterrent unilaterally would not lead others to do the same. Instead, it would undermine our security and that of our Nato allies.

"A world where the UK’s potential adversaries have nuclear weapons and the UK (and Nato) does not, is not a world in which you and your family are safer."

What is the chain of command for potential Russian nuclear strikes?

A 2020 document called "Basic Principles of State Policy of the Russian Federation on Nuclear Deterrence" says the Russian president takes the decision to use nuclear weapons.

A small briefcase, known as the Cheget, is kept close to the president at all times, linking him to the command and control network of Russia's strategic nuclear forces. The Cheget does not contain a nuclear launch button but rather transmits launch orders to the central military command - the General Staff.

Do the Russians have rules on launching nuclear weapons?

The 2020 doctrine presents four scenarios which might justify the use of Russian nuclear weapons:

— the use of nuclear weapons or weapons of mass destruction against Russia or its allies;

— data showing the launch of ballistic missiles aimed at Russia or its allies;

— an attack on critical government or military sites that would undermine the country's nuclear forces response actions;

— the use of conventional weapons against Russia "when the very existence of the state is in jeopardy"

How would the West respond?

According to Dr Patricia Lewis writing on the Chatham House website, it is more likely that Russia would use a "lower yield" nuclear weapon as part of its invasion of Ukraine rather than target another Nato ally directly.

In this instance, Dr Lewis says that a Russian deployment of such weapons would likely be picked up by US satellite systems. If such a circumstance comes to pass, Nato allies would then need to decide whether or not to intervene and prevent Russia launching such missiles.

Writing on the Chatham House website, Dr Lewis adds: “There are enormous risks associated with this decision, as to attack might precipitate a far worse attack from Russia and could be characterised as an act of aggression by NATO rather than of pre-emptive defence. However, not to pre-empt leaves Ukraine or other countries open to nuclear weapons explosions with the possibility of hundreds of thousand dead, depending on the target.”