More than one in five of Britain's 67,400 frontline soldiers is either injured, medically unfit or filling a vital administrative or training role which excludes them from taking part in operations in Iraq or Afghanistan, according to Ministry of Defence figures obtained by The Herald.

The 20.7% shortfall increases pressure on the 53,400 who are deployable on a rolling tempo of six-month combat tours which dictates that 7100 will be in Helmand and 4100 in Basra until further notice.

It also places further strain on the Territorial Army, itself 9000 soldiers short of full strength, which has sent 14,000 of its available 21,069 fully trained part-timers to plug gaps since 2003.

For every man or woman serving on the frontline, another has just returned and is recuperating and retraining while a third is preparing to go. The cycle means the basic requirement is for more than 33,000 fit soldiers at any given time, not counting the 1200 men in two reserve battalions on standby for emergencies in the Balkans and elsewhere.

As revealed by The Herald last week, more than 10% of the British infantry supposed to go to Iraq on the last deployment to Basra were unfit and could not be sent.

The four infantry battalions of the mechanised brigade based at Basra's airport complex, including the Royal Scots Borderers and the Scots Guards, had 250 of the 2200 soldiers they did have declared unfit to deploy, forcing the Army to fill gaps in their ranks from other regiments and reservists.

The new figures are even worse than those presented earlier this month by Patrick Mercer, Tory MP and former infantry commander, who claimed up to 7000 front-line troops - one in 14 of the Army's overall numbers - were unavailable because of sickness or injury exacerbated by overstretch.

Colonel Bob Stewart, who commanded a battalion in Bosnia, said: "Our troops need more rest, more training and more time off between operational deployments. The bottom line is that there are just not enough of them for what they are being asked to do."

British forces have suffered more than 4000 casualties in Iraq and Afghanistan, the equivalent of one in 24 of all trained soldiers, since the start of the "war on terror" in 2001.

The MoD said: "Non-deployable personnel will include those who are medically unfit and those filling key roles in support of families of those service personnel who are not deployed.

"Additionally, there are elements of the Army that are deemed as being in non-deployable posts employed in essential roles often directly supporting operations within the training organisations, staff headquarters and the regional forces command structure."

Meanwhile, dozens of US troops in Iraq fell sick at bases using "unmonitored and potentially unsafe" water supplied by the military and a contractor once owned by Vice-President Dick Cheney's former company, the Pentagon's internal watchdog said.

A report said soldiers experienced skin abscesses, cellulitis, skin infections, diarrhoea and other illnesses after using discoloured, smelly water for personal hygiene and laundry.

The Pentagon's inspector general found water quality problems between March 2004 and February 2006 at three sites run by contractor KBR Inc, and between January 2004 and December 2006 at two military-operated locations. It said KBR's water quality "was not maintained in accordance with field water sanitary standards" and the military-run sites "were not performing all required quality control tests".

KBR said its water treatment "has met or exceeded all applicable military and contract standards. KBR's commitment to the safety of all of its employees remains unwavering".

KBR is a former subsidiary of Halliburton Co, the oil services conglomerate that Cheney once led.

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