The Army sacked more than five times as many soldiers for cocaine abuse in the 18 months to June this year as it lost in combat in Iraq and Afghanistan combined, The Herald can reveal.

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And those figures might be "the tip of the iceberg" masking a wider problem linked to the intensity of operations in the two war zones, according to a new independent study of Ministry of Defence figures.

An analysis by the Royal United Services Institute (RUSI) published today shows that between 500 and 600 trained troops were dismissed after testing positive for the drug during that period, while 100 died fighting insurgents in the two war zones.

Professor Sheila Bird, a senior scientist and statistician with the British Medical Council who carried out research for the study, said yesterday that cocaine use among soldiers had quadrupled since 2003.

"But it could just be the tip of the iceberg. If there has been no change in the way random tests are carried out, then the rise in positive results between January 2006 and June 2007 coincides with a period of major operations," she said.

"Cocaine shows up in urine for only two or three days after use, as opposed to cannabis, which is detectable in urine samples for two to three weeks.

"Infrequent cocaine use has a high chance of going undetected in random sampling, so the observed cocaine-positive tests could, in fact, be masking infrequent cocaine use rates that are two to three times higher.

"As things stand, the Army is losing almost a cocaine battalion' a year for disciplinary reasons at a time when it cannot afford to shed trained manpower."

Professor Bird's study showed that detected military use of cocaine had gone from being 20 times lower than comparable civilian rates in 2003 to just six times lower last year.

The government has meanwhile refused on cost grounds to say whether the random tests which cover up to 100,000 service personnel a year across the armed forces are targeted on units after weekends or mass leave periods.

Last month, 17 members of the Argyll and Sutherland Highlanders (5th battalion, Royal Regiment of Scotland) tested positive for cocaine after a post-exercise leave in Cancun, Mexico.

Professor Bird added: "If there have been targeting changes to the test regime, then this could explain up to three times more positive returns without indicating any underlying increase in infrequent, recreational use.

"If that is not the case, then the results are genuinely concerning. In that case, it could mask a true rate of up to three times the discovered levels."

Christianne Tipping, RUSI's defence management analyst, said recruitment and retention problems meant that the Army might have to reconsider its zero-tolerance policy on Class A drug use.

"The MoD needs to be sure that its approach still constitutes the best way of managing the problem of drug abuse. There could be an argument for looking at a more pragmatic strategy so that recruitment difficulties in certain key areas are not compounded by high discharge rates."