Sir Sherard Cowper-Coles said that the effort to stabilise and rebuild the war-torn country was a "marathon rather than a sprint".

He also accepted that Britain should have stepped up its diplomatic and development commitment in Afghanistan earlier.

"The task of standing up a government of Afghanistan that is sustainable is going to take a very long time," Sir Sherard told BBC Radio 4's Today programme.

"It's a marathon rather than a sprint. We should be thinking in terms of decades."

He insisted: "We're not (talking) about a long-term military presence but we're serious about a long-term development presence, because this country does matter to us and to the region in so many ways."

The British embassy in Kabul has been expanded significantly over recent months, including the appointment of Sir Sherard - seen as a high-flier at the Foreign Office.

There is speculation that it could become even bigger than the diplomatic outpost Washington.

"Maybe we should have raised our game earlier, but now we have," Sir Sherard said. He insisted that although the Nato forces had been responsible for some civilian casualties, most Afghans "want us here".

"Their worry is not about us staying. It's about us going," he added. "The great thing about the Taliban is that they haven't been reading their Chairman Mao. They don't have popular support."