The Army sacked almost 1800 soldiers - the equivalent of three battalions of infantry - for drug abuse in the past three years, The Herald can reveal.

The men were dismissed after testing positive for Class-A narcotics such as cocaine and heroin during random blood and urine samplings carried out as part of the MoD's drug-testing policy.

A further 251 who tested positive for narcotics classified as "recreational" were allowed to remain in service provided they took part in a rehabilitation programme on a last-chance basis.

The summary discharge of 1791 soldiers comes at a time when British troops are stretched to the limits in Afghanistan and Iraq and the infantry is short of more than 3000 front-line soldiers.

A total of 3762 service personnel were discharged between 2001 and 2005 - the equivalent of six-and-a-half battalions - for failing compulsory drug tests or dealing in illegal substances.

The figures obtained by The Herald also reveal the number using Class A drugs such as heroin has doubled from 260 to 520 since 2003.

Those sacked since 2005 include 17 soldiers from the Argyll and Sutherland Highlanders, now 5th battalion, Royal Regiment of Scotland, who were tested after returning from a post-operational break in the Mexican resort of Cancun last November.

An MoD spokesman said: "Drugs use is incompatible with life in the armed forces and is not tolerated. No soldier wants to be on operations beside someone with a loaded weapon who has a history of substance abuse and whose judgment and reactions might be flawed or so slow as to be life-threatening."

A spokeswoman for the Army said: "Recruits are the product of the society from which they enlist. It's not surprising that many join up thinking that recreational drug use is somehow normal."

Jim Panton, chief executive of the Poppy Scotland charity, said: "Drugs are a comparatively minor problem in the military. The real scourge is alcohol abuse."