Air Chief Marshall Sir Jock Stirrup, who retires as Chief of the Defence staff on Friday, told the BBC that a significant cut in numbers would not come until the end of Prime Minister David Cameron's 2015 deadline for the end of combat operations.
"I suspect that our reduction is going to be fairy rapid and steep towards the end of that period rather than gradually over the next four to five years," he said. "It would be wrong to suggest that from next year you could start to see significant reductions in numbers of British forces; I really do not think that would be sensible."
US officials said the Taliban had been able to "re-establish and regenerate" after coming under attack by the addition Nato troops.
"The insurgency seems to be maintaining its resilience," a defence official told the Washington Post newspaper. "For the senior leadership not much has changed. At most we're seeing lines of support disrupted but its temporary."
Mikhail Gorbachev, the former Soviet leader who pulled the Red Army out of Afghanistan in 1979, added his own warning that victory for Nato in Afghanistan was "impossible" but advised President Barack Obama to press on with the withdrawal of US troops from the middle of next year.
"Victory is impossible in Afghanistan. Obama is right to pull the troops out. No matter how difficult it will be," he said. "But what's the alternative – another Vietnam? Sending in half-a-million troops? That wouldn't work."
William Hague, the Foreign Secretary, dismissed Mr Gorbachev's warnings, telling the House of Commons that Nato had the right strategy to secure British national security interest in Afghanistan although he predicted attacks by the Taliban were continuing to escalate.
"The Government is confident that we have the right military strategy in place and the right number of troops in Afghanistan," he said. "However we must expect levels of violence to remain high, and even increase, as Afghan and Isaf forces tackle the insurgency."
A UN report issued in September said there had been 95 per cent increase in assassinations of Afghans and an 82 per cent increase in road side bombings over the past year.
Moscow was driven out of Afghanistan after losing a decade-long battle to crush a vicious insurgency backed by the West. In recent weeks, the Kremlin has indicated it is prepared to return to Afghanistan.
Mr Hague welcomed Russian offers to assist the Nato campaign in Afghanistan.
Separately Osama bin Laden, the al-Qaeda leader, issued a new warning of attacks on French targets. An audio tape broadcast by al-Jazeera said French involvement in Afghanistan and its ban on the burka made the country an enemy of his followers. "As you wrongly have decided that you have the right to ban Muslim women from wearing the veil, is it not our right to drive out your conquerors by killing them?" he said. "The way to protect your security is to bring your tyranny against our nation to an end, most importantly to withdraw from the damned war of (former US President George) Bush in Afghanistan."
