The Territorial Army has shrunk to fewer than 20,000 trained soldiers - more than one-third below its bottom-line operational requirement as the "reinforcement of necessity" to back up the overstretched regular forces, The Herald can reveal.

While the part-timers should be able to field 30,274 men and women, only 19,940 have completed basic training and have the necessary military skills for Iraq or Afghanistan, according to Ministry of Defence figures.

It also means more than 1100 experienced reservists have quit since March, when "bayonet strength" was 21,069.

More than 17,000 individual tours have been completed by TA personnel since the start of the Iraq war in 2003. However, as The Herald revealed two weeks ago, a group of fewer than 2500 part-timers accounted for more than one-third of the total operational stints between them over the past five years.

The MoD waived rules saying no part-time soldiers should serve more than 12 months in every three years to allow eager volunteers - mainly students, the self-employed or unemployed - to return repeatedly to operations in Afghanistan and Iraq.

While 2000 citizen soldiers have done at least two frontline stints, 400 others have completed three or more tours in Basra or Helmand. It means that 11% of the overall force has deployed an average of three times each, while the remaining 18,600 members of the trained TA strength account for just 11,000 tours.

The Reserve Forces Act of 1976 states that no citizen soldier can be called up compulsorily for more than a maximum of 12 months every three years, but does not cover willing volunteers.

The act applies only to those called up for compulsory service but can be used by individuals who opted willingly for a first deployment to postpone further involuntary mobilisation.

Including three reserve SAS troopers killed by a Taliban bomb in Afghanistan in June, 12 TA soldiers have died in action so far. Two Scots killed in Iraq are among them.

A new national reservist policy due to be announced in October is widely expected to include a legal obligation to accept operational duty at least once every six years, or every three years if required for specialist roles such as battlefield medicine.

A six-month deployment involves being absent from home, family and a civilian job for up to nine months, including pre-operational training and post-operational leave.

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

Senior military commanders predict that a British garrison will be needed there for at least a generation.

There are currently 603 TA members in Afghanistan and 217 in Iraq. Another 850 have been called up for tours due to begin in October.