The UK presence in Afghanistan will need to remain for decades to help rebuild the country, British ambassador Sir Sherard Cowper-Coles has said.
Nato's International Security Assistance Force includes UK troops
"The task of standing up a government of Afghanistan that is sustainable is going to take a very long time," he told BBC Radio 4's Today programme.
He added that the Afghan people wanted the UK presence to help resist the Taleban and develop the country.
Extra diplomatic staff are being deployed to Afghanistan this year.
"The message we are getting, the message I had only last week down in Helmand from the people of the villages there, was, 'Please protect us from the Taleban,'" said Sir Sherard.
"Their worry isn't about us staying, it's about us going; about us not finishing the job of standing up the police, standing up the security forces, standing up the judicial system, putting schools and hospitals in place."
This week, BBC News is taking an in-depth look at the challenges facing Afghanistan's people and the peacekeepers.
Stories include: the state of the Taleban; corruption; the drugs problem; and attacks on schools.
He added: "They remember the Taleban - they have had a test-drive of Taleban rule and if there is one thing they are clear about it's that they do not want to return to the dark days of medieval Taleban rule."
The BBC learned in January that the government planned to send as many as 35 extra diplomatic staff to Afghanistan.
The priorities would be to combat corruption, help build government institutions in the south and to tackle the production of opium, the Foreign Office said.
The number of UK troops in Afghanistan is also being boosted to about 7,700 this year. They will be mainly based in the volatile Helmand province, where they have been fighting the Taleban.
BBC world affairs editor John Simpson said the British embassy in Kabul was likely to become the UK's biggest anywhere.
"It's a huge commitment," he said.
"The fact that Sir Sherard is here as ambassador is itself a sign of the Foreign Office's determination to upgrade its whole representation in Afghanistan.
"He's a big hitter in the diplomatic service."
Sir Sherard said: "We are going to win this, but it's going to take time. It's a marathon rather than a sprint - we should be thinking in terms of decades."
Meanwhile, a soldier serving in Afghanistan has complained that forces are fighting the Taleban without enough men or equipment, making them vulnerable to attack.
The 23-year-old told BBC Radio 4's The World Tonight of a number of missions which had gone dangerously wrong.
In one instance, 16 men were sent to clear an enemy compound holding as many as 100 Taleban fighters, but their trucks failed to operate in the rough terrain.
They then had to carry heavy packs in 50C heat without enough water before being ambushed. They had no back up.
The Ministry of Defence has admitted the armed forces are stretched and says it hopes troop reductions in Iraq, Bosnia and Northern Ireland will ease the situation.