General Sir David Richards told The Times of London there is "absolutely no chance" of NATO giving up its mission there.
"The Army's role will evolve, but the whole process might take as long as 30 to 40 years," he said. "I believe that the U.K. will be committed to Afghanistan in some manner -- development, governance, security-sector reform -- for the next 30 to 40 years."
Britain now has about 9,000 soldiers in Afghanistan, second only to the United States among NATO contingents. Three paratroopers were killed Friday by a roadside bomb, bringing the number of casualties to 195.
Richards said the major goal in the short run is to build up the Afghan Army and police. He said success in that effort is "our route out militarily."
But he said it is important to let Afghans know Britain will not abandon them even if it ends its military commitment.
Liam Fox, shadow defense secretary for the Conservative Party, said the country cannot afford a 30- to 40-year military commitment.