Many will head for Taliban heartlands in the south of the country, where American forces hunting Osama Bin Laden suffer regular casualties. No peacekeeping forces have ventured there before.

The British will be based in the lawless southwestern province of Helmand, a stronghold of the Taliban and centre of the opium trade that has long been a no-go area for aid agencies.

They will also operate for the first time in the southern city of Kandahar, taking over from US forces whose main objective has been to track down Al-Qaeda and Taliban forces.

Until now the Nato-led International Security Assistance Force (ISAF) has stayed in Kabul and the north, avoiding Pashtun areas of the south and east, despite appeals from President Hamid Karzai to extend its operations across the country.

“We know that going south means we will have to fight in those areas and we will fight,” said a senior British officer involved in the planning.

The UK-led Allied Rapid Reaction Corps will take charge of the peacekeeping operation next May and the number of British troops in Afghanistan is expected to rise from 1,100 to about 5,000.

The military leadership is keen to run its own show after playing a subordinate role to the Americans in Iraq. Tackling the opium trade will be a major priority.

“From a military perspective, we see Afghanistan as something that can be resolved but we need to . . . realise that counter-narcotics is central,” said one official. “If we don’t do something, it will turn into another Colombia.”

Last week Lt Gen John McColl, the first commander of ISAF after the fall of the Taliban in 2001, visited Kabul to discuss the British deployment with Afghan officials. In February he was appointed by Tony Blair as his special envoy for Afghanistan and is highly regarded in the country. The plans, which will be finalised only after the general election, will mean withdrawing forces from Iraq, where 9,000 British troops remain.

A fall in the number of terrorist attacks in Iraq has boosted confidence that British troops will be able to start withdrawing early next year. By contrast, attempts to create a national army in Afghanistan are still at an early stage.

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