Bob Ainsworth, the Defence Secretary, has admitted the Government has pushed the armed forces "too hard" and that Britain only has scope for a "relatively small" troop increase in Afghanistan.
But he was rebuked by ex-Army chief General Sir Mike Jackson, who said the UK should act "decisively" at a critical time for the campaign.
Mr Aisnworth's comments came at a Labour Party conference fringe event in Brighton.
Mr Ainsworth said the military had been "running hot" when major operations were taking place in both Iraq and Afghanistan.
He said: "To keep 12,000 (troops) abroad, deployed in operational theatres, was a lot.
"We were pushing people too hard in many ways. It's not sustainable - sustainable for a short period of time but not for a long period."
Referring to reports that the US will ask Britain to increase its forces in Afghanistan as part of a surge to improve security, Mr Ainsworth said: "We can only make a relatively small part of that as the UK. So others have got to step up to the plate."
Sir Mike insisted that the UK did have the capacity to increase troop levels over a short period.
"When so much blood and treasure has already been expended, if this is a decisive year we should act decisively," he told the event in Brighton.
"The Secretary of State talked of the difficulties of sustaining such numbers. Indeed over a number of years I would agree. But if this is a critical year then I think we can do more on that time basis, and indeed should if that is what the commanders on the ground believe is required."
He went on: "The decision is not one of military capacity. It is a matter of the politics of the situation."
Too bloody right.