The departing commander of British forces in Afghanistan says he believes the Taleban will never be defeated.

Brigadier Mark Carleton-Smith, the commander of 16 Air Assault Brigade, whose troops have suffered severe casualties after six months of tough fighting, will hand over to 3 Commando Brigade Royal Marines this month.

He told The Times that in his opinion, a military victory over the Taleban was “neither feasible nor supportable”.

“What we need is sufficient troops to contain the insurgency to a level where it is not a strategic threat to the longevity of the elected Government,” he said.

The brigadier said that his troops had “taken the sting out of the Taleban” during clashes in Helmand province in southern Afghanistan, but at a heavy cost. His brigade suffered 32 killed and 170 injured during its six-month tour of duty. The 2nd Battalion the Parachute Regiment alone lost 11 soldiers, most of them killed by roadside bombs or other explosive devices.

The brigadier’s grim prognosis follows a leaked cable by François Fitou, the deputy French Ambassador in Kabul, claiming that Sir Sherard Cowper-Coles, the British Ambassador, had told him the strategy for Afghanistan was “doomed to failure”.

In the cable, Mr Fitou told President Sarkozy that Sir Sherard believed “the security situation is getting worse, so is corruption and the Government has lost all trust”. He said Sir Sherard had told him Britain had no alternative but to support the US, “but we should tell them that we want to be part of a winning strategy, not a losing one. The American strategy is doomed to fail.”

Brigadier Carleton-Smith admitted that it had been “a turbulent summer” but he said that the Taleban were “riven with deep fissures and fractures”.

He added: “However, the Taleban, tactically, is reasonably resilient, certainly quite dangerous and seems relatively impervious to losses. Its potency is as a force for influence.”

He indicated that the only way forward was to find a political solution that would include the Taleban. The Government of President Karzai has launched a reconciliation programme, although the hard core of Taleban commanders is thought to be implacably opposed to any compromise. Efforts are being focused on the so-called “tier-two” and “tier-three” Taleban, who are perceived to be less ideologically intransigent.

The brigadier said that in the areas where the Government had no control, the Afghan population was “vulnerable to a shifting coalition of Taleban, mad mullahs and marauding militias”. In other areas, however, progress was being made and children were going back to school. “We are trying to deliver sufficient security for a degree of normalisation,” he said.

The British commander said that more foreign trainers were needed to help to build up the competence of the Afghan National Army. He suggested that they would be provided by the Americans. He said that there had been a government vacuum for 30 years, and even now the central Government in Kabul did not view Helmand as a key province. He said that in some areas the Afghan people were now beginning to shift their allegiance towards traditional power structures “rather than the shadowy and illegal structures” of the Taleban and the warlords.

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