Chief of the Defence Staff Air Chief Marshal Sir Jock Stirrup said it was "crucial" for the armed forces to experience a reduction in the tempo of operations over the coming period, in order to recover from several years of overstretch.
His comments come as US President-elect Barack Obama prepares to order a significant increase in troop numbers in Afghanistan in the hope of finally quelling the Taliban insurgency in the country.
Foreign Secretary David Miliband today indicated that the UK, whose 8,100 troops in Afghanistan make it the second largest contributor to the international force, will expect other Nato countries to take up a bigger share of the burden in any US-led "surge".
And he appeared to rule out the commitment of British troops to the peace-keeping force in the Democratic Republic of the Congo, insisting: "That is not on the agenda."
Sir Jock Stirrup said he was "optimistic" that 2009 will see a "significant reduction" in the UK's 4,000-strong contingent in Iraq, as the fundamental change of mission promised by Prime Minister Gordon Brown comes into effect.
But he cautioned that this will not mean that thousands more servicemen and women become available for deployment to Afghanistan and other hotspots around the world.
Sir Jock told BBC1's Andrew Marr Show: "I have said for a very long time that the British armed forces are stretched. We are doing more than we are structured and resourced to do in the long term. We can do it for a short period, but we can't continue doing it ad infinitum.
"We have to put ourselves back into balance. It is crucial that we reduce the operational tempo for our armed forces.
"So it can't be - even if the situation demanded it - just a one-for-one transfer from Iraq to Afghanistan. We have to reduce that tempo."
Sir Jock warned that the struggle in Afghanistan was "a marathon, not a sprint", adding: "We need to be there at the finishing line."
Success in Afghanistan will not be achieved militarily but by successfully implanting good governance and political stability in the country, as well as training the national armed forces to defend their own country.
"I am a little nervous when people use the word 'surge' as if this were some sort of panacea," he said. "What we are quite clear about is that we need more military force in Afghanistan - the Nato combined statement of requirement has yet to be fully met.
"We welcome more military force being sent to Afghanistan. Everybody needs to do their share, we are very clear on that.
"In the context of what we are doing in Iraq and Afghanistan, we are shouldering a burden which is more than we are able to shoulder in the long term, so we expect the others to take up their share of that burden."
Asked whether Britain had the capacity to offer peace-keeping forces for the Congo, Sir Jock replied: "The United Nations has 17,000 troops on the ground, so there are more than enough military there already."
Shadow foreign secretary William Hague said he believed the bulk of British troops would be pulled out of Iraq next year but warned that Britain made a "disproportionate contribution" to the Nato effort in Afghanistan.
He told Sky News' Sunday Live: "I think by this time next year British troops will have been pulled out of Iraq, other than some who may remain for training and other special operations.
"But I think the bulk of the troops will have been pulled out and we will be supportive of that in the opposition."
Asked whether those troops would then be sent to Afghanistan, Mr Hague said: "The British Army is very overstretched and Britain makes a disproportionate contribution to the Nato effort in Afghanistan.
"So I think we would all take some persuading that there would have to be a much larger British contingent there - there's already a very large British contingent."
He added: "We do need the rest of Nato to play its part in Afghanistan and undoubtedly it seems that Barack Obama does intend to send larger US forces and that is part of what is necessary in Afghanistan."
Asked if President-elect Obama's plans to step up the pace of operations in Afghanistan will require an increase in the size of Britain's commitment there, Mr Miliband told the Andrew Marr Show: "Not necessarily, no."
He added: "We will look at what new American deployments are going to be. President-elect Obama has said he wants two new brigades to go in, there are an extra 1,500 French troops and the Germans are increasing the number of their troops.
"As the second-largest contributor of troops in Afghanistan, the first thing we say is that we don't want to bear an unfair share of the burden
"The second thing we say is that more foreign troops on their own are not going to provide the answer in Afghanistan. It needs to be an approach that combines a serious security presence with the development of the country...
"It's got to be a civilian surge as well as a military surge. That is the lesson from Iraq as well as Afghanistan."
Mr Miliband said he expected 2009 to see Britain's relationship with Iraq develop into a normal relationship between friendly states.
"The Prime Minister has made it very clear that there will be a fundamental shift in the nature of our relationship with the Iraqi Government after the spring of next year," he said.
"That fundamental shift happens because Britain's job - which now is about training the 14th Division of the Iraqi Army - is now over.
"We will have a normal relationship with the Government of Iraq. It will be a relationship between allies and friendly nations, but it won't involve the sort of troop deployments that we have got there at the moment."
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