The world stands at the dawn of a “third nuclear age” in which Britain is threatened by multiple dilemmas, the head of the armed forces has warned.

But alongside his stark warning of the threats facing Britain and its allies, Admiral Sir Tony Radakin said there would be only a “remote chance” Russia would directly attack or invade the UK if the two countries were at war.

The Chief of the Defence Staff laid out the landscape of British defence in a wide-ranging speech, after a minister warned the Army would be wiped out in as little as six months if forced to fight a war on the scale of the Ukraine conflict.

The admiral cast doubt on the possibility as he gave a speech at the Royal United Services Institute (Rusi) defence think tank in London.

We are at the dawn of a third nuclear age... defined by multiple and concurrent dilemmas, proliferating nuclear and disruptive technologies and the almost total absence of the security architectures that went before

He told the audience Britain needed to be “clear-eyed in our assessment” of the threats it faces, adding: “That includes recognising that there is only a remote chance of a significant direct attack or invasion by Russia on the United Kingdom, and that’s the same for the whole of Nato.”

Moscow “knows the response will be overwhelming”, he added, but warned the nuclear deterrent needed to be “kept strong and strengthened”.

Sir Tony added: “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.”

The first nuclear age was the Cold War, while the second was “governed by disarmament efforts and counter proliferation”, the armed forces chief said.

He listed the “wild threats of tactical nuclear use” by Russia, China building up its weapon stocks, Iran’s failure to co-operate with a nuclear deal, and North Korea’s “erratic behaviour” among the threats faced by the West.

But Sir Tony said the UK’s nuclear arsenal is “the one part of our inventory of which Russia is most aware and has more impact on (President Vladimir) Putin than anything else”.

Successive British governments had invested “substantial sums of money” in renewing nuclear submarines and warheads because of this, he added.

The admiral described the deployment of thousands of North Korean soldiers on Ukraine’s border alongside Russian forces as the year’s “most extraordinary development”.

He also signalled further deployments were possible, speaking of “tens of thousands more to follow as part of a new security pact with Russia”.

Defence minister Alistair Carns earlier 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 comments reported by Sky News, Mr Carns, a former Royal Marines colonel, said Russia was suffering losses of around 1,500 soldiers killed or injured a day.

“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 Rusi.

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, the minister for veterans and people, said the UK needed to “catch up with Nato allies” to place 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.”