Military commanders plan to increase Britain’s frontline fighting units in Afghanistan by at a least a quarter amid signs the war against the Taliban is intensifying.

General Sir Richard Dannatt, the head of the army, has ordered an extra infantry battalion of frontline troops including Gurkhas to take on the Taliban in war-torn Helmand province, according to sources.

Four battalions, one each from the Coldstream Guards, the Yorkshire Regiment, the Royal Gurkhas and the Royal Marines have been formally “warned” that they should be ready to deploy to Helmand this autumn. They are set to replace the three fighting battalions currently in theatre.

The decision to deploy the Gurkhas will send a signal to the Taliban commanders that British forces will not flinch in their determination to press the Taliban back high into the mountains. More than 1,000 of them are reported to have been flown from their base in Brunei for training in Britain.

Des Browne, the defence secretary, announced last February that troop numbers would increase by 1,400 to 7,700 this summer. But the new plans are likely to see the overall deployment increase by as many as 800 more fighting men, bringing the total commitment to 8,500.

The MoD said last week that it could not comment on any plans for new troop deployments. A spokesman said details of any new deployment would first be announced by Browne in a statement to parliament. That was expected in the autumn, he said.

“We do not discuss the details of deployments before the secretary of state makes his announcements to parliament,” he said.

When John Reid, then defence secretary, first announced the deployment of a British battle-group to Afghanistan last year, he said he hoped they would be able to leave “without a shot being fired”.

But in the last 18 months more than 50 British soldiers have died in the region, many in Helmand province, a notorious Taliban stronghold.

Reports of the planned deployment come just days after Sir Sherard Cowper-Coles, Britain’s ambassador in Kabul, predicted that British involvement in the region would last at least 30 years. He described the offensive as “a marathon, rather than a sprint”.

Patrick Mercer, the Tory MP and former army colonel said: “The decision to send further combat forces to Afghanistan represents a serious and necessary escalation in combat power. The problem, of course, is that this imposes further strain on an already overstretched army. There needs to be a greater number of combat troops recruited and trained to ease the strain.”

The additional deployment to Afghanistan comes as Britain prepares to reduce its military presence in Iraq. Britain now has 5,500 troops based in Basra. Some military sources have suggested this number will be cut to 1,500 next year but the MoD has declined to comment on this, dismissing it as speculation.

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