Gen Richards said the Afghan Army and police force must be built up
The UK's commitment to Afghanistan could last for up to 40 years, the incoming head of the Army has said.
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.
Gen Richards' comments came as it emerged that three servicemen, from the Parachute Regiment, had been killed north of Lashkar Gah, in Helmand province, southern Afghanistan, on Thursday afternoon.
Their deaths - in an attack on a Jackal armoured vehicle which left a colleague critically injured - take to 195 the number of British troops killed in Afghanistan since 2001.
The Army has suffered its heaviest losses of the entire campaign in recent weeks, but its soon-to-be chief said he strongly believed the campaign was "winnable".
"Demanding, certainly, but winnable," he said.
He added: "The end will be difficult to define; it won't be neat and clear-cut like the end of some old-fashioned inter-state war might have been."
He said it would take "a long time and considerable investment", adding: "We must remember, though, that we are not trying to turn Afghanistan into Switzerland."
Gen Richards said great efforts must be made to expand the Afghan National Army and build up the police force - only then could the UK's military role "decline".
"I believe that the UK will be committed to Afghanistan in some manner - development, governance, security sector reform - for the next 30 to 40 years," he said.
"It is not just reconstruction; jobs and simple governance that works are key, and there has to be a strong reconciliation element to the latter."
For the Tories, Mr Howarth said: "It would not be fair to those who have given their lives for this conflict to say, 'actually, we need to find out how we can scuttle out of here as quickly as possible'."
However, he said the general was not suggesting maintaining the current level of operations for the next 40 years.
Gen Dannatt has called for the government to commit more troops and equipment to Afghanistan, but Gen Richards said he would not be presenting a "shopping list" to ministers.
However, he said the Army and the government needed to "continue to respond flexibly and quickly to the evolving requirements of our campaign in Afghanistan".
Labour MP Mike Gapes, chairman of the Commons foreign affairs committee, said there were "serious questions" to be asked about why other Nato countries were "not pulling their weight".
Mr Gapes asked: "Why are only a few countries taking the major burden of this?
"That is the big issue for the international community - not just for the UK."