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UK 'may have 40-year Afghan role'
Gen Sir David Richards, who takes over on 28 August, told the Times that "nation-building" would last decades.
Troops will be required for the medium term only, but the UK will continue to play a role in "development, governance [and] security sector reform," he said.
Shadow defence minister Gerald Howarth said the UK had to be there long-term to achieve its objectives.
Gen Richards commanded 35,000 troops from 37 nations when he was head of Nato's International Security Assistance Force in Afghanistan between May 2006 and February 2007.
He will take over from Gen Sir Richard Dannatt as the UK's chief of the general staff.