Nato's commander in Afghanistan has said the country's citizens may start supporting the Taleban unless their lives improve in the next six months.
Gen Richards believes success over the winter is crucial
Lt Gen David Richards, a British officer, said the country was at a "tipping point", warning that up to 70% of Afghans could switch their support.
They might prefer the "austere and unpleasant" life under the Taleban to five more years of fighting, he said.
He was speaking a day after Tony Blair pledged full support for UK troops.
Gen Richards said: "If we collectively ... do not exploit this winter to start achieving concrete and visible improvement, then some 70% of Afghans could switch sides."
The British general wants about 2,500 additional troops to form a reserve battalion to help reconstruction and development efforts.
He said southern Afghanistan was "broadly stabilised" and insisted that Nato had to take advantage of military victories.
Gen Richards said: "We have created an opportunity, following the intense fighting that left over 500 militants dead in the southern provinces of Kandahar and Helmand.
"If we do not take advantage of this, then you can pour an additional 10,000 troops next year and we would not succeed because we would have lost by then the consent of the people."
Addressing military personnel on the fifth anniversary of operations in the country on Saturday, Mr Blair pledged "every support and every protection".
He said this included providing more armoured vehicles and more helicopters.
In September alone, seven soldiers died in Afghanistan as a result of hostile action and 14 died when a RAF Nimrod crashed after a suspected technical fault.
Opinion polls have also shown there is significant support for a British troop withdrawal from the country.
Nato has taken charge of the country's eastern provinces, which have been under the control of US forces since the Taleban were ousted five years ago.
The alliance's International Security Assistance Force already commands troops in the north, west and south of Afghanistan, as well as Kabul.
It means that some 12,000 US soldiers have now come under the command of Gen Richards.