In an advisory paper reported by the media on Wednesday, they sounded the alarm that the size of the British army may already be beneath "national critical mass".
The warning comes as the military establishment is didpleased with Prime Minister Rishi Sunak's failure to increase defense spending in the new budget.
"British Army credibility has been weakened by 20 plus years of hollowing out and diminished fighting power. Army morale is fragile," they 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."
Defense Secretary Grant Shapps had previously warned that the UK was in a "pre-war world" amid Russia's special military operation in Ukraine.
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