The head of the Army has warned that any gap between military resources and planned capabilities caused by spending cuts "could become quite dangerous, quite quickly".
Chief of the General Staff General Sir Peter Wall said imposing further efficiencies while the impact of previous deep spending cuts on the armed forces were still being absorbed would be "very disruptive".
His comments came with the Ministry of Defence under pressure to deliver its share of the £11.5 billion spending cuts across Government demanded by Chancellor George Osborne in his spending review.
Defence has some protection in the 2015/16 review, to be unveiled on June 26, with a guaranteed 1% increase in equipment budgets from 2015, but Defence Secretary Philip Hammond has still been asked to find a 5% cut in other spending.
However, senior MoD sources claimed a deal with the Treasury was now close that would not involve any further reduction in military numbers or capability.
Sir Peter was interviewed as part of the Britain's Last War documentary broadcast on Sky News Thursday night.
Presenter Jeff Randall asked him: "The forthcoming spending review seems sure to bring further cuts to the defence budget, and the arms budget, leaving a considerable gap between planned capabilities and available resources. How dangerous is that gap?"
He replied: "I think it could become quite dangerous, quite quickly."
He told the programme: "We have got to the point in a number of parts in our set up where we can't go any further without seriously damaging our professional competence and our chances of success in the battlefields of the future.
"It would be a brave claim to say an organisation can never make more efficiencies but we do need the time to let our new structures bed in, for those efficiencies to get delivered.
"Imposing more on us now, before the last round of efficiencies have really materialised properly in a balanced way, would be very disruptive."
The UK's most senior officer in Afghanistan, Lieutenant General Nick Carter, told the documentary that politicians should "look themselves in the mirror each morning" and ask if the risks of cuts were manageable.
"At the end of the day our politicians need to determine what they want the Army to do. And if they determine that the Army's going to do less, then it's reasonable for them to reduce it still further," he said.
"But commitments have got to match resources, if you like, in our military terminology, ways and means have to equate to the ends that you want to push the army towards.
"We are bound as military people to point out the risks, during the course of this, to our political masters and ultimately it's down to them to look themselves in the mirror each morning and determine whether or not those risks are manageable."
Shadow defence minister Kevan Jones said: "The Government have cut the Army without a replacement plan in place. The country will worry about strategic shrinkage by stealth.
"The Government's defence plans rest on doubling the number of reserves and yet there is a recruitment crisis and employers have not been properly engaged. Ministers should put as much energy into recruiting reservists as they do into sacking soldiers.
"This stark warning underlines that the Government have taken a huge gamble. The country will worry that reduced numbers in the absence of reform will limit Britain's ability to meet our ambitions in the world. Our services' morale is continuing to fall.
"There are worrying skills shortages in the Army. Rather than salami slicing, strategic purpose should drive the capabilities we have.
"Labour has argued for our forces to be reformed around the principles of adaptability and prevention, to focus on training others to defend themselves and trained in stabilisation and language skills. Ministers, by contrast, have focused on savings over strategy."
An MoD spokeswoman said: "Like all Whitehall departments we are currently negotiating our financial settlement for the 2015-16 Spending Review.
"Although no final decisions have been taken we have been clear that we would first and foremost seek to find genuine efficiencies that would enable us protect front-line capabilities and protect military manpower numbers.
"Whilst this process is ongoing it would be inappropriate to comment further."