Russia has stepped up its threats of nuclear conflict in the event of a NATO military intervention in Ukraine, with Putin warning of a “lightning-fast” response from Moscow. NATO is committed to being a “nuclear alliance” based on the principle of deterrence and requires all member states to subscribe to its nuclear component.

But it is clear that “should the fundamental security of any NATO ally be threatened, NATO has the capabilities and the will to impose costs on the adversary that would be unacceptable and far outweigh the benefits that any adversary could hope to obtain”.

Yet John Erath, senior director of policy at the Center for Arms Control and Non-Proliferation, told Express.co.uk that NATO nations can only repel an “extremely limited attack”.

If Russia were to launch a ballistic missile, “the United States and NATO maintain missile defenses designed to stop an extremely limited attack,” Erath said.

Mr Erath, who previously served on the US National Security Council, said that would include an accidental missile launch.

But these defensive measures would not withstand any nuclear threat Vladimir Putin might unleash, he added.

Mr Erath said: “These defenses would have no possibility of stopping the type of massive attack that Russia is capable of launching.”

Britain, a NATO member, lacks nuclear defense capabilities, an international relations expert previously told Express.co.uk.

Professor Andrew Futter from the University of Leicester explained that active nuclear defense has long since disappeared from the UK agenda.

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“Since the start of the Cold War, the UK has effectively relied on deterrence because defense (either active missile defenses or passive civilian defence) is hugely expensive and in some cases impractical.

“British policy is based on the threat of nuclear retaliation after a strike.

“No adversary would risk attacking the UK with nuclear weapons because they would be attacked with nuclear weapons in response.”

The UK does not have the US ‘Doomsday’ aircraft or Russia’s ‘dead hand’ launch system, he added.

The ‘Doomsday’ aircraft is a modified Boeing-747, adapted to become an airborne military command center in the event of a nuclear attack.

The “dead hand” mechanism, also known as perimeter, means that an automatic retaliatory strike could be launched by the Russian command even if all of its top leaders were killed.

This system “is based on signals transmitted by a series of rockets that could be fired if the Russian leadership were killed”, Prof Futter explained.

He added ominously: “As far as I know the UK does not have that capability – ‘nuclear war’ would be fought from a bunker under Whitehall and from the patrolling submarine.”

Hidden beneath the corridors of Whitehall is Pindar, the UK’s crisis command center for a nuclear strike.

But Professor Futter described how, in the event of a nuclear attack, the UK would be ‘destroyed’ as a ‘functional state’.

He added: “There is very little defense against a large nuclear explosion on British soil, let alone multiple nuclear explosions.”