The Territorial Army was the reserve of last resort for most of its history but has become the reserve of necessity since 2003 to plug the gaps in Britain's threadbare front line.

Now the citizen-soldier organisation which can date its distant ancestry back to the Saxon fyrd defeated defending a bloody ridge at Hastings in 1066 will celebrate its official centenary this week in the looming shadow of fundamental change to its structure and function. The TA's first units were "stood up" on April 1, 1908, just in time to train a generation of enthusiastic amateurs to face and thwart the might of imperial Germany at Mons and Le Cateau, the battles which stalled the Kaiser's intended knockout blow against France in 1914.

The latest recruits train to fight the Afghan Taliban and Iraq's insurgents in a form of warfare their forebears could not have imagined as they struggled to squeeze off 10 aimed rounds a minute from bolt-action rifles in the killing-fields just across the Channel.

The more recent Cold War days when the TA was widely regarded as a uniformed and paid drinking club are thankfully gone. It was never a fair description for citizen-soldiers who would have made up almost one-third of the British Army of the Rhine if the Soviet army had ever rolled west across the north German plain as the trigger for world war three.

The popular, disparaging image of the "weekend warrior" has now been replaced by the harsh realities of providing manpower to flesh out a regular army which is under-strength, overstretched and fighting two simultaneous wars on a shoestring peace-time budget.

More than 15,000 part-time soldiers have already served six-month tours of duty in hot, sandy places since 2003. Eight, including two Scots, have died and many more have been wounded.

About 1000 Scots TA members have been on front-line operations. Those deployed include volunteers from Scotland's two part-time infantry battalions, 6th and 7th Scots, which are relatively well recruited with 825 out of the 967 soldiers they should have on their books.

The TA across the UK is currently 9000 below its trained requirement at 21,069 men and women and haemorrhaging the very people it needs to retain, largely because of the prospect of repeated compulsory mobilisations for Iraq and Afghanistan.

The main factors affecting recruitment and retention are understandable family resistance to extended separation and fear of damage to civilian careers caused by the prospect of an endless succession of compulsory mobilisations.

The only part of the nationwide organisation consistently bucking the outflow trend is Scotland, where volunteers are filling the ranks to an average of 80% or more of their manning levels, fully 10% above the national average.

The Ministry of Defence has now announced a radical shake-up of the TA in a move aimed at producing a deployable force of fully trained soldiers ready for call-up every three to five years.

Between the needs of the two war zones and peacekeeping in the Balkans, the MoD needs to mobilise between 600 and 800 TA soldiers every six months.

Most are sent as individual replacements to flesh out regular infantry battalions which are an average 90 men below "bayonet strength" across the Army.

The MoD says its review of reservists will "focus on operations", but denied that TA soldiers will be dismissed simply because they refuse to go to war. Insiders say the likeliest outcome will be a two or three-tier TA, separating those who agree to be mobilised for overseas tours as and when needed.

Lieutenant- General Nick Parker, the Inspector General of the Territorial Army, said yesterday: " The TA has had to adapt and change."