Speaking separately at the same think tank, the head of the armed forces said it was important to note there was only a “remote” chance that Russia would launch a “significant direct attack or invasion” on the UK, or other Nato member.

Admiral Sir Tony Radakin added that Russia “knows the response would be overwhelming, whether conventional or nuclear”.

But he added that it showed the need for nuclear deterrence to be “kept strong and strengthened,” warning that the world was entering a “third nuclear age” following the Cold War and subsequent period of disarmament.

This new age would be defined by the “almost total absence of the security architectures that went before,” and more countries having nuclear weapons.

He added that China’s increasing nuclear weapons stockpile meant the United States could face a “two-peer challenge” from Beijing and Moscow, with both countries possessing significant arsenals.

Army reservists serve in their spare time, getting paid to train outside their main jobs.

Carns said the reality of wars such as the one being fought in Ukraine was that they were “attritional in nature”.

He also said the UK needed to “catch up with Nato allies” by placing a greater emphasis on its reserves.

A Ministry of Defence spokesperson said the UK’s armed forces were “amongst the best in the world and offer a 24/7 defence of the UK, operating alongside our allies and partners to prepare for any event”.

“The Strategic Defence Review will look at the threats we face and the capabilities we need so that our Armed Forces are better ready to fight, more integrated and more innovative,” they added.

“Our Reserves are an essential and extremely valued element of the Armed Forces and the contributions they make to our resilience and our ability to call on additional personnel when required are vital.”

Earlier, the prime minister’s official spokesman said the Budget had “invested billions of pounds into defence”.