The two wars have left the Forces “stretched beyond the capabilities we have,” Air Chief Marshal Sir Jock Stirrup said.
It is the first time the most senior officer in the British military has expressed such grave doubts about the struggle faced by troops fighting wars on two fronts.
And his comments came as it was announced another British soldier has been killed in a firefight with the Taliban in Afghanistan, taking the UK death toll in the country to 107.
Highlighting the pressures prolonged operations are putting on soldiers, sailors and airmen and their relatives, the Chief of the Defence Staff also called for services families to be given priority access to public services.
Ten British service personnel have been killed in Afghanistan in little over two weeks, fuelling fears that the UK has been sucked into a long and unwinnable struggle in the country.
Sir Jock admitted that Britain could faces decades more involvement in Afghanistan, which he called a “mediaeval” state lacking even basic government structures.
Labour ministers have repeatedly denied suggestions that the two wars in Iraq and Afghanistan have left the Forces overstretched, and opposition MPs said Sir Jock’s candid remarks have made that position untenable.
There are 4,000 British troops in Iraq and numbers in Afghanistan will soon exceed 8,000.
A resurgence in violence in Basra forced the Government to postpone an Iraqi withdrawal earlier this year, and despite ministers’ hopes, there is still no clear timetable to withdraw forces from Iraq.
Speaking to journalists at Westminster, Sir Jock gave a stark assessment of the pressure the two deployments have placed on the Forces, saying that Britain simply lacks the men and material required.
“We are not structured or resourced to do two of these things on this scale on an enduring basis but we have been doing it on an enduring basis for years,” Sir Jock said. “Until we get to the stage when one of them comes down to small scale, we will be stretched beyond the capabilities we have.”
He said: “That is not what we are structured for nor is it what we plan for. We are very stretched at the moment. Until we get down to one operation at this scale, we are always going to be stretched.”
Sir Jock’s blunt comments about the strain on the Forces are his most critical yet on the issue. Last May, he told MPs that the military was “very stretched” by the two wars, and warned: “In the not-too-distant future we need something to change.”
Since then, he CDS has faced criticism from some serving service personnel for not being more openly critical of Government defence policy, and some officers have compared him unfavourably with Sir Richard Dannatt, the Chief of the General Staff, who has said the ongoing operations could “break” the Army.
Earlier this month it was confirmed that Sir Richard will not be promoted to replace Sir Jock as CDS and the air chief marshal will be asked to stay on in post for another year.
Sir Jock was speaking after being asked why, after seven years in Afghanistan and five in Iraq, British forces still do not have enough helicopters.
Nick Harvey, the Liberal Democrat defence spokesman, said Sir Jock had been “admirably straightforward about the question of overstretch Â we are simply not geared up for two operations on this scale.”
Patrick Mercer, a Tory MP and former Army commander, said: “The message from the CDS is clear: our forces are overstretched and are finding it close to impossible to sustain the numbers they are being required to.”
The air chief marshal insisted that British troops are well-equipped and insisted that Britain has made “considerable progress in Iraq.”
But he admitted that Afghanistan “looks considerably messier than Iraq does at the moment.”
Hours later, the MoD confirmed that 2nd Battalion the Parachute Regiment, had been killed during a firefight in Helmand province.
The MoD said: “The soldier was on a deliberate operation against the Taliban in the Upper Sangin Valley when he was fatally wounded.”
The Government last week announced another troop reinforcement in Afghanistan, where British forces are battling Taliban-backed militias and drug gangs to bring stability to the desperately poor country.
Sir Jock said the West is trying to solve “a mammoth problem in Afghanistan.”
He said: “This is not something that could be done in one, two or three years because we are talking about a country that is essentially medieval, that has very little in the way of infrastructure, very little in the way of human resource, that has an endemic culture of corruption.”
He added: “In terms of developing the country from an almost medieval status, that has to be an enterprise of decades.”
Ministers have been under growing political pressure over the treatment of military families, with the Conservatives drawing up plans to give service spouses and children priority access to public services like the NHS.
Sir Jock said that current arrangements put service families at a disadvantage, with many struggling to get access to services when they move to a new area as a result of a new military posting.
That must change in order to stop key personnel leaving the military, he said.
He said: “My objective is to see first of all as an absolute starting point that there is a level playing field for our people. But then actually from my perspective there should be a little bit more.
“Actually they should be treated a little bit better because of the service they give to their country and the sacrifices they make.”
He did not elaborate on how that could be done, saying more details will emerge in a Ministry of Defence command paper later this year.