Britain's armed forces are "very stretched" by operations in Afghanistan and Iraq, the chief of the defence staff has told MPs.
British forces could withdraw from some areas in Iraq
Air chief marshal Sir Jock Stirrup told the defence committee there was "not much more left in the locker".
Commitments had been greater and gone on longer than planned, he told MPs.
Shadow defence secretary Liam Fox said the "variety" of concerns expressed by Sir Jock "underlines the fact our armed forces are teetering on the brink".
The Liberal Democrats said the chief of the defence staff's comments further highlighted the need to withdraw British troops from Iraq by October.
Giving evidence to the defence select committee, Sir Jock said: "We are able to sustain what we are doing at the moment but in the not too distant future we need something to change."
Sir Jock said he had been encouraged by recent announcements on troop withdrawals in Iraq.
But even if withdrawals continued as expected, Britain would not be able to fight another war for "some years" because of the impact operations in Iraq and Afghanistan have had on training.
"If we are able to reduce the operational tempo, as we hope and intend to, over the next 18 months - and we achieved the first very slight loosening of the screw just recently - then we should be in a position to reinstate some of this training.
"But quite clearly we are not going to be in the business of engaging in large-scale, high-end war fighting operations for some years to come.
"Because not only do we need to conclude what we are currently doing, or at least bring the levels down, we will need to train a new generation of people."
All three armed forces were short of manpower, said Sir Jock, and personnel were being kept away from their home bases for longer, without leave, than recommended.
If operations continued at the current level for longer than expected, there would have to be a major rethink, he added.
"We will either at some stage, in the not too distant future, need to reduce the overall level of commitment, or we are going to think about the overall force structure."
The armed forces could cope with redeployment to Afghanistan, but there was "not much more left in the locker," he told MPs.
"We do have other capacity for certain emergencies but it's pretty limited," he added.
He also suggested Britain would have to increase defence spending.
Britain was likely to face high demands on its armed forces in the years ahead because of continuing global instability and that was going to "need the right level of investment," said Sir Jock.
Asked if he believed Britain would have to spend a greater share of its national income on defence, a smiling Sir Jock said: "You might very well think that. I couldn't possibly comment."
He said the government would decide in the next 12 months on future military spending priorities, and it would become clear then whether there was a need for a more wide-ranging defence review.
Commenting afterwards, Dr Fox, for the Conservatives, said: "Air Chief Marshal Sir Jock Stirrup joins the list of the senior serving military officers to express grave concerns over the state of our armed forces.
"The air chief marshall supports our view that the expectations for deployments in Iraq and Afghanistan were rose-tinted to say the least.
"For that, our troops are now paying the price. We are now faced with inadequate training, poor quality accommodation, an end to drugs operations in the Caribbean, and a failing air bridge to Iraq."
He said the "variety" of concerns expressed by Sir Jock "underlines the fact our armed forces are teetering on the brink".
And he added: "The question now is: when will the government join the rest of us in the real world?"
In January, Prime Minster Tony Blair said Britain had to decide what sort of defence role it wanted to play after he had stepped down.
"My choice is for armed forces that are prepared to engage in this difficult, tough, challenging campaign, to be war fighters as well as peacekeepers," he told an audience of servicemen in Plymouth.
But he said there had to be a national debate on increasing defence spending.