He emphasised that British troop involvement, currently 9,000-strong, should only be needed for the medium term, but insisted that there was “absolutely no chance” of Nato pulling out. “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.

Three paratroopers from the Special Forces Support Group were killed yesterday when their Jackal armoured vehicle was blown up by a roadside bomb north of Lashkar Gah.

Their deaths raised the total number of servicemen and women who have died in Afghanistan to 195 since 2001. Four US Marines were also killed in a separate incident. Across the border in Pakistan, officials said that the leader of the Taleban in the country had been killed in a missile strike but analysts said that his death would have little impact on the battlefields of Afghanistan.

Liam Fox, the Shadow Defence Secretary, said that 30 to 40 years in Afghanistan was “unaffordable”.

“Any idea of maintaining military involvement for that length of time is not a runner. It would require a total rethink of our foreign and security policy,” he said. The military campaign in Afghanistan has already cost British taxpayers more than £5 billion.

General Richards said: “We need now to focus on the expansion of the Afghan National Army and Afghan National Police. Just as in Iraq, it is our route out militarily, but the Afghan people and our opponents need to know that this does not mean our abandoning the region. We made this mistake once. Our opponents are banking on us doing it again, and we must prove them wrong,”

Of the Taleban, he said: “We can and are outfighting them.”