Vladimir Putin knows he would be “overwhelmed” and lose a World War, the head of the British military has declared.

Admiral Sir Tony Radakin, the Chief of the Defence Staff, claimed there is “only a remote chance of a significant” confrontation between Britain and Russia.

But Sir Tony said the Kremlin “knows the response would be overwhelming, whether conventional or nuclear”.

However, the head of the military warned the World has moved into a “third nuclear age” as Moscow makes “wild threats”, China builds up its arsenal, Iran pursues an atomic bomb and North Korea builds up its erratic missile programme.

He told a RUSI conference: “There is only a remote chance of a significant direct attack or invasion by Russia on the United Kingdom.

“It is the same for the whole of NATO. Russia knows the response would be overwhelming, whether conventional or nuclear.

“The strategy of deterrence by NATO works and is working. But it has to be kept strong and strengthened against a more dangerous Russia.

“This assessment is separate to actions short of war, where Russia is clearly active and eager to both challenge and undermine the international system – but whilst reckless and unacceptable, these threats are manageable and reflect the increasingly pariah status of Russia.

“Our unity, cohesion, economies can withstand this pressure. We should not fret nor buckle.

Russian tyrant Mr Putin has made repeated threats to use nuclear weapons if Western nations arm Ukraine with long-range missiles.

Moscow used an intercontinental ballistic missile to strike a target in Dnipro, and the tyrant has threatened to strike the Ukrainian capital – Kyiv – with Oreshnik missiles.

And Sir Tony said this is symptomatic of a shift in attitudes towards nuclear weapons.

He said: “The first nuclear age – the Cold War – was defined by two opposing blocs governed by the risk of uncontrollable escalation and the logic of deterrence.

“The second nuclear age by disarmament efforts and counter-proliferation. But we are at the dawn of a third nuclear age which is altogether more complex.

“It is defined by multiple and concurrent dilemmas, proliferating nuclear and disruptive technologies, and the almost total absence of the security architectures that went before.

“From Russia we have seen wild threats of tactical nuclear use, large scale nuclear exercises and simulated attacks against NATO countries, all designed to coerce us from taking the action required to maintain stability.

“China’s nuclear build up poses a two-peer challenge to the United States. Iran’s failure to cooperate with the IAEA is a concern, and North Korea’s ballistic missile programme and erratic behaviour presents a regional and, increasingly, a global threat.

“Nuclear non-proliferation has been one of the great successes of international security since the end of the Second World War but is now being challenged.

“It has been successful because of states that took their international responsibilities seriously, and those, like Britain and the United States, who were willing to extend their nuclear umbrella to allies and partners and guarantee their security.

“This must continue. The UK’s nuclear deterrent is the one part of our inventory of which Russia is most aware and has more impact on Putin than anything else.”

Sir Tony’s intervention came after a defence minister claimed the British Army would be wiped out within six months if it fought a major war.

Alistair Carns said a rate of casualties similar to Russia‘s invasion of Ukraine would lead to the army being “expended” within six to 12 months.

He said it illustrated the need to “generate depth and mass rapidly in the event of a crisis”.

“In a war of scale – not a limited intervention, but one similar to Ukraine – our army for example on the current casualty rates would be expended – as part of a broader multinational coalition – in six months to a year,” Mr Carns said in a speech at a conference on reserves at the Royal United Services Institute defence think tank in London.

He added: “That doesn’t mean we need a bigger army, but it does mean you need to generate depth and mass rapidly in the event of a crisis.”

Official figures show the army had 109,245 personnel on October 1, including 25,814 volunteer reservists.

Mr Carns said: “The reserves are critical, absolutely central, to that process.

“Without them we cannot generate mass, we cannot meet the plethora of defence tasks.”

Mr Carns, the minister for veterans and people, said the UK needed to “catch up with Nato allies” to place a greater emphasis on the reserves.

The Prime Minister’s official spokesman said Defence Secretary John Healey had previously spoken about “the state of the armed forces that were inherited from the previous government”.

The spokesman said: “It’s why the Budget invested billions of pounds into defence, it’s why we’re undertaking a strategic defence review to ensure that we have the capabilities and the investment needed to defend this country.”