Army chief Sir Peter Wall warns cuts could be dangerous
Gaps between military resources and planned capabilities caused by spending cuts "could become quite dangerous, quite quickly", the Army's head says.
Chief of the General Staff General Sir Peter Wall said further efficiencies would be "very disruptive".
Gen Wall was speaking ahead of the Whitehall spending review on 26 June.
Prime Minister David Cameron said "difficult decisions" had been taken but there would be no further cuts to numbers in the armed forces.
Asked on Sky News documentary Britain's Last War about further cuts to the defence budget, Gen Wall said: "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, Lt Gen Nick Carter, told the documentary that politicians should "look themselves in the mirror each morning" and ask if the risks of cuts were manageable.
Chancellor George Osborne is hoping to save £11.5bn across government in his spending review for 2015-16.
There will be a guaranteed 1% increase in defence equipment budgets but Defence Secretary Philip Hammond is expected to have to find a 5% cut overall.
Speaking at a Downing Street news conference ahead of next week's G8 summit in Northern Ireland, the prime minister said the UK's defence budget was still the fourth largest in the world.
He said: "While we have had to make difficult decisions, we have very strong armed forces and ones the whole country can be proud of.
"We're not going to be making further cuts to the numbers of our army, navy or air force, they know what they have available.
"But no department can be excluded from being efficient, from saving money, from making sure we get the best possible value for every pound that we take from the taxpayers and spend."
Chief Secretary to the Treasury Danny Alexander said plans announced by Mr Hammond to shave millions off the MoD's procurement budget were an example of how savings could be made.
Speaking to BBC Radio 4's The World At One, he added: "What we all want to do is to maintain the front-line military capability that matters to this country, that is the purpose that we have set out as a government.
"The point I am making is... there are plenty of areas to look which mean that one doesn't have to look in the areas that Peter Wall and I are both concerned about."
In a separate interview with Sky News, Mr Alexander said: "In a department where there are more horses than tanks, there is room for efficiency savings."
Labour said the government was taking a "huge gamble".
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 MoD said it was currently negotiating its financial settlement for the spending review.
An MoD spokeswoman said: "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."