The UK is "effectively incapable" of launching an effective military offensive against a belligerent foe like Russia, experts have claimed.
Defence cuts resulted in "hollowing out or deletion of the army's deployed capabilities" and rendered the force unprepared if the country was dragged into a conflict, the Centre for Historical Analysis and Conflict Research said.
Following two days of seminars, retired and serving military generals warned the risk of losing the army's one remaining fighting division in an afternoon will "weigh heavily" on commanders.
Their report reflected what the UK's capabilities would be if Russia flexed its muscles and attacked a Nato ally in Europe.
The British Army stands 83,000, its lowest level since the Napoleonic Wars at the beginning of the 19th century. It has shrunk by 20,000 from when the coalition government came to power almost seven years ago.
Experts warned the reduced size meant the risk of losing the army's one remaining fighting division in an afternoon will "weigh heavily" on commanders. The Times reported that a paper prepared by the think tank concluded while there were no imminent threats to the UK there were "plausible scenarios" that could see Britain embroiled into conflict if one country attacked another.
"This raises an important question: is the British Army ready for such a possibility? If one merely sees preparedness through net manpower and kinetic force capacity, the answer might be a simple 'no': the British Army is at its smallest and has faced years of budget cuts," the paper said.
The army is down to just one deployable division, down from nine during the Cold War and 90 during World War One.
The last time a division was deployed was in Iraq in 2003 and military personnel now believe Britain would be unable to commit anything more than a brigade of 10,000 troops.
The Ministry of Defence said "the army is ready and capable of deploying a potent, large-scale, war-fighting force at divisional level with sufficient notice".