Britain's ability to "deliver and sustain" an effective fighting force in the face of a "competent" enemy like Russia has been "effectively removed", according to the Army's think-tank.
A paper from the Centre for Historical Analysis and Conflict Research (CHACR), seen by the Sunday Times, says defence cuts have caused the "hollowing out or deletion of the Army's deployed capabilities".
It also warns that the danger of its one fighting division being wiped out in an afternoon will "weigh heavily" on commanders.
The paper, which was put together by serving and retired army officers and academics after two days of seminars last summer, said there are "a few plausible scenarios" which would see Britain dragged into a war after an attack on another country, despite there not being an "immediate risk of a direct attack by a foreign state". It continued:
"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."
Under 2015's Strategic Defence and Security Review it was announced the Army's sole "war-fighting division" would be expanded from up to 30,000 troops to up to 50,000, and equipped with more heavily-armoured vehicles.
The paper warns, however, that with this division making up over half of the Army's 82,000 regular soldiers, there would be "political pressures" to safeguard it if the UK was at war. It said:
"The 'prospect of losing the division in an afternoon' will weigh heavily on the chain of command . . . as politicians appreciate the stakes involved in committing the division to battle."
It added that the Army must be able to "regulate how much risk" the division is exposed to during war "unless we are prepared to lose it".
The seminars also discussed how the Army had been "distracted by counter-insurgency operations in Afghanistan for a decade" and that it now "lacks some of the tools that would be required against a threat from the East".
Ben Barry, a former brigadier who works at the International Institute for Strategic Studies think-tank, said there is a "clear and present" danger of a "military miscalculation" with Russia, and that the Army "might struggle" to deploy as many troops as it did to Iraq in 2003.
There was also a "consensus" that Britain doesn't have enough large aircraft and ships "to transport a large-scale fighting force and its stores". According to the paper, the military would rely on allies or commercial companies to get more than one of the division's three brigades to war:
"The ability to get an entire division's component parts swiftly and safely into a theatre of operations is identified as a key concern."
It comes after the Army moved tanks through the Channel Tunnel to test its ability to move equipment by rail last week.
Challengers were taken to France and back on trains during the exercise; reportedly in case of an invasion of Eastern Europe.
The paper, meanwhile, also warned that the military "does not yet comprehend" the cyber-threat and the danger of future enemies engaging in "hybrid warfare", while "air defence" - the ability to protect troops from enemy aircraft - is described as a "capability gap".
A Ministry of Defence spokesperson, however, said:
"The Army, in line with the Strategic Defence and Security Review of 2015, is ready and capable of deploying a potent, large scale, war fighting force at divisional level with sufficient notice."