In many circumstances we will need to be able to strike very rapidly

MOD discussion paper

"It is of the nature of terrorists... that those threatening the UK will be hard to find, may present only fleeting opportunities to attack them and can often choose inaccessible areas for training and planning," the discussion paper says.

"In many circumstances, therefore, we will need to be able to strike very rapidly."

Defence officials recognise that if British soldiers are to be deployed abroad, pursuing the 'war on terror', radical thinking is needed about how to best organise and equip them.

Officials have also been considering the implications for "homeland defence" if increasingly more time is to be spent away from base.

Defence Secretary Geoff Hoon announced in the wake of the 11 September attacks that a new chapter would be added to the 1998 Strategic Defence Review.

The MoD has said that air defences had already been strengthened to counter the threat of a civilian airliner being hijacked and used in a suicide attack.

Defence challenge

Extra police patrols since 11 September have stretched forces' budgets, particularly in London where there is a preponderance of targets.

The Territorial Army's force of 40,000 volunteers, many with local knowledge about key sites, could be vital in guarding installations.

Police budgets have been stretched by extra patrols since 11 September

Mr Hoon told BBC Radio 4's Today programme that the proposals set out ideas for the way reservists could help meet the challenges posed following the terror attacks on the US.

"One of the important questions is whether we want to use regular members of the armed forces for the purpose, particularly when they are so extensively engaged overseas," he said.

Conservative MP Patrick Mercer, who sits on the Commons defence select committee, welcomed many of the proposals.

But he said the UK would need to spend a lot more money if the plans were going to work.

"The Americans, who of course have just suffered a huge catastrophe, are looking at spending an extra $48bn on their defence budget whilst there are no plans currently for any expansion of Britain's defence budget."

Mobile troops

He said the UK's credibility was not helped by news that the RAF's Number Five Squadron - tasked with the defence of London - was on verge of disbandment.

As well as an enhanced role for the TA in defence within the UK, Mr Hoon has already made clear the greater emphasis on "agile and adaptable" armed forces capable of global deployment at short notice.

As part of the review, officials have been looking at expanding the number of highly mobile troops like the Paras and the Royal Marines - training some infantry units to play a similar role.

The review has involved five main strands: the changed military environment since 11 September, international relations and defence diplomacy, homeland security, conventional military operations and special operations.

Urgent response

Each strand was headed by a "two-star officer" or equivalent ranking civilian official from the MoD.

The need for an urgent response means that officials hope that the review will be completed by April.

Conservative frontbench defence spokesman Desmond Swayne, himself a member of the territorials, said his colleagues would welcome a greater involvement in homeland defence, but not at the expense of their current role as backup for regular army units.

The government had run down the Territorial Army and would now need to repair the damage, he maintained.