The unit, nicknamed the “X-men”, has already begun recruiting and is expected eventually to comprise some 600 men and women from all three armed services and the intelligence agencies.

Particular efforts will be made to recruit people of Arabic appearance in addition to members of ethnic minority communities and Muslims. The unit would be expected to operate around the world as well as to counter the terrorism threat in Britain itself.

Much of the core of the unit will be made up of undercover surveillance operators who have honed their skills fighting terrorists in Northern Ireland.

More than 150 members of the 14th Intelligence and Security Company, have already left Northern Ireland and are forming the nucleus of the new unit.

The company was involved in the bugging of Gerry Adams, the Sinn Fein president, during the 1998 Good Friday peace negotiations.

Other Northern Ireland veterans who are experts at undermining terrorist groups using moles and informers are also likely to be recruited.

The move was hinted at in last week’s announcement by Geoff Hoon, the defence secretary, on the restructuring of the armed forces. “We are increasing the strength of our special forces and investing in new equipment for them,” he said.

The government will make a separate move in its anti-terrorism efforts tomorrow when ministers launch a leaflet telling members of the public how they can prepare for a terrorist attack. The advice, to be sent to all homes in England and Wales over the next few weeks, will suggest people “stay upwind” of any release of gas or radiation and remain indoors in the event of nuclear detonation.

If they believe they have been infected by biological warfare agents, they are advised to ring NHS Direct rather than spread germs by going to hospital. They should “wash with soap and water” if they come into contact with a suspicious substance.

The advice is part of a £5m publicity drive by the government to prepare for the possibility of an attack by Al-Qaeda or other groups.

The leaflet — provisionally entitled Go In, Stay In, Tune In — suggests people should stay indoors and listen to the BBC for further instructions in an emergency. Households are advised to stock up on tinned food, a first aid kit, a battery-powered radio and a mobile phone and charger.

The terrorist threat, particularly with the Olympics approaching, is still considered severe. It has emerged that the US Olympic committee was among leading American sports bodies that were warned last week Al-Qaeda may be planning to attack a sporting event at home or abroad this year.

A new security focus on sports stadiums has been prompted by a growing US conviction that Al-Qaeda will attempt to disrupt either the Athens Olympics or the US presidential campaign with an attack designed to inflict mass casualties.

“Everybody feels (Al-Qaeda) are trying to mount another attack,” said Thomas Kean, chairman of the 9/11 Commission, which last week called for a restructuring of US intelligence agencies. “And everybody feels that they are doing their best to make it chemical, biological or nuclear because it kills more people and that’s their goal.

“We are in danger of letting things slide. Time is not on our side.”

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