SBS troops are in the front line of a battle to stop hundreds of Taliban fighters crossing Pakistan’s border into Afghanistan. “They are very, very busy, going out, taking the Taliban on head-to-head,” said one source.

Only two of the four squadrons of the SBS — the navy’s equivalent of the SAS — are designated for land warfare. These will alternate constantly in Afghanistan until the new squadron is formed next year.

The plan to expand the SBS comes amid concern that other British troops operating in the southern province of Helmand may have too few infantry to deal with a resurgence of the Taliban, whose regime was ousted in 2001.

Former defence chiefs who were briefed on the deployment last month were shocked at the low number of “boots on the ground”.

“I have real, sincere concerns,” one said. “We’re involved in an unpopular war (in Iraq). I wonder whether we haven’t now accepted a mission impossible in Afghanistan.”

The 3,300-strong British force deployed to Helmand — an area twice the size of Wales — includes 600 infantry from 3rd Battalion, The Parachute Regiment, and 150 Gurkhas. Early plans provided for 2,000 infantry.

Lieutenant-General David Richards, commander of the Nato-led International Security Assistance Force, conceded last week that the number “looks horrendous” but emphasised that two-thirds of Helmand was desert.

He added: “Every general would like more troops because then I could do things quicker. But I will just have to construct a campaign that reflects what we’ve got.”

Privately, former defence chiefs have expressed concern over peacekeeping rules of engagement that prevent troops opening fire unless they are sure that they are under threat — and over a lack of clarity about the mission’s purpose.

When John Reid, the former defence secretary, announced the deployment, he said that it would not conduct counter-terrorism operations. But the British are taking a full part in Operation Mountain Thrust, launched last week, which is intended to drive the Taliban out of southern Afghanistan. Afghan and coalition troops yesterday killed about 45 suspected insurgents during attacks on Taliban camps there.

Liam Fox, the Conservative defence spokesman, said:

“Our biggest worry is that there is not sufficient flexibility to maximise the success of the mission and minimise the risk to our troops.”

Richards warned that the international community had to work more closely together to satisfy basic needs or risk failure. “People don’t have water or sewerage and a quarter of children die by the age of five, yet we have been focusing money on things like civil service reform and gender rights,” he said. “This is a just conflict and we can’t afford to fail.”

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