The Territorial Army is short of 7,000 troops and is losing 600 volunteers a month, a BBC investigation reveals.
Recruitment for the TA is a 'priority'
About 10% of British forces in Iraq are provided by the TA, whose part time soldiers support regular army units.
The minister responsible for the TA, Don Touhig, says recruitment is now a priority - but he denies the Iraq conflict is putting recruits off.
The Radio Five Live Report says some TA soldiers feel undervalued while others have been sacked while away on duty.
TA soldiers, who combine their service with civilian jobs, train for 27 days a year but are finding themselves called up for six-month tours of Iraq.
BBC World Affairs Correspondent Jonathan Charles said without them, overstretched regular units would be unable to fulfil the UK's commitments in Southern Iraq.
But the TA only has 39,000 troops, 7,000 less than required.
Mr Touhig said he does not believe the shortage is caused by people not wanting to serve in Iraq but is the result of more complex factors.
As well as feeling undervalued, some former TA members say they are sometimes unable to get the healthcare they need after they return to Britain.
Others say they were fired while serving abroad.
Mr Touhig has promised to look at all the problems.