Government cuts to the Army have not been thought through and could leave Britain dangerously exposed in the event of a future crisis, MPs have warned.
The Commons Defence Committee said the restructuring progamme - known as the Army 2020 plan - was driven by the need to fit a "financial envelope" rather than a proper assessment of potential threats.
It urged the Ministry of Defence (MoD) to draw up contingency plans for a rapid recruitment programme in case there was urgent need for more troops to deal with an emergency.
Under the Army 2020 plan, the size of the Regular Army is being slashed from 102,000 troops to 82,000 while the numbers of part time reservists is to be expanded to 30,000 by 2018.
In a hard-hitting report, the committee said the rationale for the plan remained untested while the "high level of change" involved - at a time when troops were still engaged in Afghanistan - could "compromise" its ability to respond to emergencies.
It complained at the "apparent lack of consultation and involvement" of the head of the Army, General Sir Peter Wall, who was simply told by the senior civil servant at the MoD what the future strength of the Army would be.
Despite the radical nature of the changes, they were not even discussed by the National Security Council - which the Prime Minister chairs - even though they represented a further 12,000 reduction in its overall strength compared with the plans set out in the 2010 Strategic Defence and Security Review.
The committee warned that if ministers attempted to make any further cuts, the whole plan would "unravel" completely.
"We note that the Secretary of State for Defence (Philip Hammond) accepts that Army 2020 was designed to fit a financial envelope. We are concerned that this consideration took primacy over the country's abilities to respond to the threats, risks and uncertainties contained in the National Security Strategy," it said.
"We remain to be convinced that the Army 2020 plan represents a fully thought-through and tested concept which will allow the Army to counter emerging and uncertain threats and develop a contingent capability to deal with unforeseen circumstances."
The committee said the plan even contained a built-in financial incentive not to commit the troops on operations, as reservists were only cheaper to employ as long as they were not actually called up.
"This will need to be closely monitored. It would be unacceptable if the UK decided not to take part in any action because of the cost of deploying reservists," it said.
"The Government must also set out its contingency plans for the rapid recruitment of Regular Army personnel should there be a need for the rapid expansion of UK armed forces."
Any failure to recruit the necessary numbers of reservists, the committee said, could leave the Army short of personnel in key supporting capabilities resulting in a loss of "real fighting power".
Mr Hammond however accused the committee of failing to recognise the need to counter evolving threats, such as potential cyber attacks, meant there had to be a shift in the balance of defence spending.
"It is not possible to maintain traditional regular forces at historic levels while also investing in countering the threats of tomorrow," he said.
"The Army 2020 structure is not simply about a reduction in size; it is a complete overhaul of how the Army works to deliver a fully-integrated force, using a better mix of regulars, reserves and contractors to get the maximum defence effect from the budget.
"The changes currently taking effect will ensure the Army has a sustainable and properly funded future, as a flexible and agile force with first class kit and worldwide reach."
For Labour, shadow defence secretary Vernon Coaker said that with official figures showing a shortfall of almost 8,000 in overall armed forces numbers, the Government should re-think the Army 2020 plan.
"Labour warned that there could be consequences for Britain's safety and security, but the Defence Secretary ignored all advice and sacked thousands of regular soldiers without the promised increase in reserve recruitment to fill the gap," he said.
"Following this latest setback, the Defence Secretary should pause his redundancy programme until it's clear that Britain's armed forces won't be left with a dangerous capability gap."
A MoD source dismissed Labour's claim of an 8,000 shortfall, saying the figures reflected an "inevitable lag" between personnel leaving and the required forces numbers being reduced as the services move to their Future Force 2020 levels.