Former security chiefs have said that Britain must "genuinely prepare for war" if the armed forces are to be a credible deterrent.
In a paper published by the New Bletchley Network, a group of key figures have called on the government to reverse two decades of hollowing out and diminished fighting power.
Some of the signees of the letter include former national security adviser Lord Sedwill, and Lord Robertson, a former Nato secretary-general.
The letter said the size of the regular army and reserve forces had shrunk "far below national critical mass" an error that must be reversed.
Lord Robertson was one of those who co-authored the report
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The paper said: "To be credible in deterrence terms we need to prepare genuinely for war and communicate this to potential foes, Nato and allies, and particularly to the British public.
“British Army credibility has been weakened by 20-plus years of hollowing out and diminished fighting power. Army morale is fragile."
In the letter, the authors also call for a New Model Army that would turn the British Army into a respected land force that prioritises preparing for high-intensity war fighting.
It comes as defence secretary Grant Shapps has described the UK as being in a “pre-war” phase. However, there is concern nothing substantial is being done to ensure the armed forces are ready to fight if necessary.
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Grant Shapps has been criticised for a lack of substantial planning
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Latest estimates showed the army will be reduced to 73,000 by the middle of the decade, the smallest size since the Napoleonic era. One senior figure in Nato told Ben Wallace, the former defence secretary, that Britain was no longer a "tier one" military power.
Despite there being no formal definition of what constitutes a "tier on" power, the Ministry of Defence has interpreted it as one having a full spectrum of military capabilities.
Serving military chiefs are divided on how to plan for a war, with some senior figures such as General Sir Patrick Sanders, the head of the army, believing the UK needs to prepare its citizens to fight and others believing that to do so would be alarmist.
One source said: "Former chiefs have a reputation for speaking up after they’ve finished and Sanders hasn’t been that guy. Sanders is a man who actually tried to get us ready for war, now. And he may be remembered in history for that."
King Charles III greets General Sir Patrick Sanders, Chief of the General Staff
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The paper was endorsed by former head of Joint Forces Command General Sir Richard Barrons, former deputy commander of the US-led coalition against Islamic State Major General Rupert Jones.
Also adding their support were General Lord Richards of Herstmonceux, the former chief of the defence staff, General Sir Richard Shirreff, the former deputy supreme allied commander in Europe, and Brigadier Nigel Hall, the former deputy UK military representative at Nato.