The Army has sacked almost two battalions-worth of soldiers for serious drug abuse since January last year, The Herald can reveal.

More than 1100 men who tested positive for Class A drug abuse have been discharged automatically at a time when British troops are stretched to the limits in Afghanistan and Iraq and the infantry is short of 3000 frontline troops.

The soldiers were found to have traces of cocaine, amphetamines or ecstasy in their systems in random blood and urine tests over the past 20 months, according to Ministry of Defence figures.

The figures also show that the numbers testing positive for Class A substance abuse have risen by 14% since 2006.

This year so far, a total of 422 positive Army tests were for Class A controlled substances, with 20 more guilty of using Class B drugs and 176 Class C. By contrast, the RAF detected only six Class A culprits and the Royal Navy 21 over the same period.

The Herald understands that the manpower problem has become so acute that commanding officers of battalions have now been given some leeway in interpreting the military's blanket "zero tolerance" policy towards drug use.

Any detection of Class A substances continues to carry the automatic loss of job and pension, but exceptions have occasionally been made for competent and previously well-behaved soldiers who have dabbled in post-operational recreational drugs.

But all units operate a "two-strikes and you're out" rule for anyone caught twice for even minor Class C abuse with cannabis.

A Ministry of Defence spokeswoman said: "Any military drug use is forbidden for obvious reasons, but it should also be remembered that relatively few of the service personnel tested are found guilty of substance abuse."