Ashdown, who was rejected last month by Afghan President Hamid Karzai for the post of senior U.N. envoy to the country, called in the Financial Times for renewed efforts to win Taliban moderates away from the insurgency.
"With fighting in Afghanistan now entering its seventh year, no agreed international strategy, public support on both sides of the Atlantic crumbling, NATO in disarray and widening insecurity in Afghanistan, defeat is now a real possibility…
"We have not lost in Afghanistan … But we will lose if we do not start doing things differently," he warned.
Ashdown said the consequences of failure in Afghanistan would be appalling.
"Global terrorism would have won back its old haven and created a new one over the border in a mortally weakened Pakistan," he said.
Ashdown called for more cooperation between international military and civilian efforts and a greater focus on governance and the rule of law across in a country where corruption and lawlessness is widespread.
Of an international security effort which has at times caused civilian casualties, he said:
"Breaking up the Taliban by winning over the moderates is a far better route to success than bombing and body counts."
U.S.-led forces toppled the Taliban government in 2001, but Taliban rebels launched an insurgency two years ago and violence has risen sharply since then.
Washington has called on NATO allies to send more troops to Afghanistan and to commit more of them to the south of the country where the Taliban insurgency is strongest.