Britain's most senior military commander in Afghanistan has warned that the war against the Taliban cannot be won. Brigadier Mark Carleton-Smith said the British public should not expect a “decisive military victory” but should be prepared for a possible deal with the Taliban.

His assessment followed the leaking of a memo from a French diplomat who claimed that Sir Sherard Cowper-Coles, the British ambassador in Kabul, had told him the current strategy was “doomed to fail”.

Carleton-Smith, commander of 16 Air Assault Brigade, which has just completed its second tour of Afghanistan, said it was necessary to “lower our expectations”. He said: “We’re not going to win this war. It’s about reducing it to a manageable level of insurgency that’s not a strategic threat and can be managed by the Afghan army.”

The brigadier added: “We may well leave with there still being a low but steady ebb of rural insurgency . . . I don’t think we should expect that when we go there won’t be roaming bands of armed men in this part of the world. That would be unrealistic and probably incredible.”

Carleton-Smith insisted that his forces had “taken the sting out of the Taliban for 2008”. But his brigade has sustained heavy losses in the southern province of Helmand in the past six months, with 32 killed and 170 injured. In an interview with The Sunday Times, he added his voice to a growing number of people arguing that the conflict in Afghanistan could be resolved only through a political settlement that could include the Taliban.

“We want to change the nature of the debate from one where disputes are settled through the barrel of the gun to one where it is done through negotiations,” Carleton-Smith said.

“If the Taliban were prepared to sit on the other side of the table and talk about a political settlement, then that’s precisely the sort of progress that concludes insurgencies like this. That shouldn’t make people uncomfortable.”

Last week Gulab Mangal, the governor of Helmand, said the Taliban controlled more than half the province despite the increased presence of British forces.

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