Labour pushed the Armed Forces “too hard” in Iraq and Afghanistan, Bob Ainsworth, the defence secretary, has admitted.
Mr Ainsworth said that trying to conduct two wars at the same time had put too much strain on the military.
Because the forces now need to recover from that pressure, he said, Britain will only be able to make a “small” extra deployment in Afghanistan.
General Stanley McChrystal, the Nato commander in Afghanistan, has said that without more troops, the international mission there will fail.
Britain has 9,000 troops in Afghanistan, and the general has signalled he will ask Britain to increase that number, perhaps by as much as 2,000.
But speaking at a debate on the fringe of the Labour Party conference in Brighton, Mr Ainsworth said that the recent strains on the Armed Forces meant that Britain will be limited in its options.
“We had, a couple of years ago, 12,000 troops abroad on operations. They were running hot. To keep 12,000 abroad, deployed in operational theatres, was a lot.”
"We were pushing people too hard in many ways. It's not sustainable. It’s sustainable for a short period of time but not for a long period without consequences."
Now, Britain is nearing the limits of its ability to send troops to the front line, the minister said.
“If Gen McChrystal asks for a substantial contribution, we can only make a relatively small contribution to that,” he said, saying other Nato members must “step up to the plate” by sending more troops and allowing them to play a full combat role.
At the debate, Mr Ainsworth was contradicted by General Sir Mike Jackson, the former chief of the general staff, who said the UK can make a larger reinforcement in Afghanistan for a short time.
“We can. When so much blood and treasure has been expended, we should act decisively,” Sir Mike said. “If this is a critical year, we can do more on that time basis.”
Sir Mike’s views reflect those of serving commanders, who are pushing for a larger deployment to Afghanistan but meeting resistance from ministers.
Opening up a gap between the main parties on the issue, Liam Fox, the Tory shadow defence secretary said that a Conservative Government would be “sympathetic to a request for an increase in the number of British troops” to train Afghan security forces.
Since taking his post in June, Mr Ainsworth has been candid about Labour’s record on defence. Last month, he told The Daily Telegraph that the British Government and public alike had not done enough to support the Armed Forces.
Nontheless, he remains under pressure over Labour’s backing for the troops. At the fringe event, he was confronted by military families telling him frontline soldiers are being left without adequate basic supplies like boots and clothing.
Christine Bonner, whose son Darren was killed in Afghanistan in 2007, said kit failures are undermining the forces morale.
“I know of families who are having to send out boots and suchlike to their sons,” she said. “If they were getting what they are supposed to be getting, morale would be much higher.”
Asked if he accepted that some soldiers have to buy their own boots and other kit, he said: “No, I don’t. No.”
Last week, The Daily Telegraph disclosed that the leaking of details of MPs' expenses earlier this year was caused byanger at the Government’s failure to support and equip British troops on the front line in Afghanistan.