Sir Richard, who served as Deputy Supreme Allied Commander Europe, said in 1997 Britain could field a war-fighting division of 20,000 well-equipped troops.

But he said that a decade of cuts had left the Army “decimated”, which now required “real emergency treatment” .

‘Ammunition in short supply’

Speaking on the BBC’s Today programme, he said; “I don’t think the British Army can field a war-fighting brigade of 5,000 without an enormous amount of time – I think even that would be really, really difficult.

“I’m sure the MoD would say yes of course we can do that. I can tell you now most of the units training on Salisbury Plain will be training without their full complement of vehicles for various reasons. 

“Ammunition is in critical short supply. Amongst other things, of course, the bottom of the barrel has been scraped to provide everything that can go to Ukraine but it needs to be replaced. It needs to be topped up and on top of that we need to increase the size of our capability to meet the challenges posed by Russia.”

A spokesman for the Ministry of Defence said: “Headline numbers do not define the British Army’s effectiveness and we are already making the Army more integrated, agile, and lethal, helping protect the UK at home and abroad.

“Our Armed Forces are always ready to protect and defend the UK and we continue to meet all operational commitments, including participating in every single Nato mission and deploying 16,000 Army personnel on Exercise Steadfast Defender, the biggest Nato exercise in a generation.”