'This could take four years'

British land forces being sent to Afghanistan face a war that may last as long as four years, Britain's military chief said.

On the day the Government gave details of the task force for the ground war, the chief of defence staff, Admiral Sir Michael Boyce, warned that Britain was facing the "most difficult military operation since the end of the Cold War, unprecedented in the problems it presents ... we are rewriting the rules as we go along".

Sir Michael prepared the public for a long war of attrition that would continue through the bitter Afghan winter and beyond, with inevitable casualties. He said: "We are in for the long haul. We can carry on until the job is done. If it takes three or four years, then it takes three or four years."

Britain will deploy 200 Royal Marines initally, specialists in mountain and winter warfare from 4 Commando Brigade with another 400 standing by. The force will also include the aircraft carrier HMS Invincible, two warships, a submarine and seven support ships; a total of 4,200 service personnel.

Because of the long-term commitment being undertaken, the forces will be rotated and reinforcements are likely to be sent, raising the combat strength to about 1,000.

The severity of the task facing the British forces was underlined by Tony Blair who warned that Osama bin Laden could be attempting to develop nuclear weapons as well as chemical and biological ones.

"If they are allowed to carry on doing this, our world will be an insecure, unsafe place and there will be no corner of the world – particularly not a place like Britain – that will be untouched by that," the Prime Minister said. "This is a battle that we have to undertake for the defence of civilised values everywhere and for the free world," he added.

The British forces will not be incorporated with any of the others in the US-led coalition, and will have specific missions. The marines are likely to take part in "smash-and-grab" raids on the Taliban leadership, Mr bin Laden and his chief lieutenants in al-Qa'ida. Admiral Boyce, speaking to defence writers, said the leadership would be hunted. "Every time they sniff the air, or look out of the parapet, they should be aware that someone is out to get them," he said.

The British forces are also likely to be used to attempt to destroy the Taliban and al-Qa'ida heroin trade which is used to bankroll terrorist acts and the stockpiling of weapons. Combat units would be used to find and destroy heroin caches.

Sir Michael gave a candid summary of the problems facing the campaign, which he described as far more difficult than the ones in the Gulf War, Kosovo and Bosnia. One of the main ones was the ephemeral nature of the enemy.

He said: "In the past we were fighting states and state machines. We are not doing that here. Osama bin Laden is tangible. But al-Qa'ida is not tangible, it's an idea, it's not something you can touch, although you can touch some of bin Laden's people."

Admiral Boyce also said that the lack of a land base and the geography created severe problems for the Afghan land campaign, the most difficult British forces had faced "since the Korean War". He agreed that, unlike during the Gulf War, Arab states were not offering bases for operations and added that the volatile political situation in Pakistan made it unsuitable to have a base there.

Sir Michael appeared to indicate a shift in the allied position on the opposition Northern Alliance. He said that British troops will work closely with the Alliance, and an occupation of Kabul by them would be acceptable as long as "they did not take Kabul in a way which turns into a bloodbath".

Yesterday, American warplanes again launched a heavy night of attacks on targets in Kabul dropping 13 bombs on the city in quick succession.

Earlier however they managed to hit warehouses of the Red Cross in the capital for the second time in 10 days, setting them on fire.

In the US, President Bush signed a sweeping anti-terrorist law, substantially broadening the powers of the FBI. Meanwhile, the anthrax scare in America continued to widen with traces of the deadly disease discovered at the largest mail distribution centre in New York, and also within the CIA, at the mail-handling building at the agency's headquarters in Langley, Virginia.

In Afghanistan, the Taliban's supreme leader Mullah Mohammed Omar appealed for Muslims to stage worldwide demonstrations within three days in support of the Taliban cause. He urged global protests within 72 hours by "those Muslims who feel that holy war is part of Islam... to support the Taliban point of view".