Despite assertions by Michael Howard, the Home Secretary, that the random testing programme is on course for success, the service gave the go-ahead for the study by the National Addiction Centre at the Maudsley Hospital, south London, earlier this month.
It follows repeated claims that inmates are turning to hard drugs such as heroin or crack cocaine which remains detectable in the bloodstream for only one to three days, compared to 20 to 28 days for cannabis.
The tests were introduced in eight pilot establishments in February 1995 and later extended to all jails. The Prison Service aims to complete 60,000 tests a year, either randomly or on suspicion.
Mr Howard told the Prison Service conference yesterday that he believed mandatory testing would become "an increasingly effective deterrent, as well as identifying drug users both for punishment and treatment."
But latest results suggest that far from stamping out the drugs problem, the tests have encouraged some prisoners to switch from cannabis to opiates.
Between March and September last year, the number of randomly selected prisoners who tested positive for cannabis fell from 25.97 per cent to 19.74 per cent, while those testing positive for opiates rose from 5.01 per cent to 6.35 per cent. Tests based on suspicion during the same period saw a similar reduction for cannabis and a rise for opiates.
A study by Sheila Gore, of the Medical Research Council Biostatistics Unit, and Graham Burd, of the Churchill Hospital, Oxford, suggested that testing had "zero effectiveness in countering addiction and disorder". And with spending running at pounds 16,000-pounds 22,000 a month for a 500-prisoner jail, the scheme cost at least twice that of a drug reduction and rehabilitation programme.
Case studies obtained by the National Association of Probation Officers (Napo) support fears that prisoners are drug switching to avoid detection. In one case, a 31-year-old at HMP Erlestoke was serving two years for drug-related offences. He failed a random test last autumn and was given the standard punishment of serving an additional 14 days. He then switched to smoking heroin to avoid further positive tests. He told his parole officer that this was not uncommon.
A 26-year-old remanded at Durham on charges of grievous bodily harm and affray was a regular cannabis user and switched to smoking heroin to beat the tests. He said heroin was as easy to obtain as cannabis.
A 24-year-old from the South-west who was sentenced to three years for various burglaries switched from cannabis to heroin and became a heavy user. He claimed his habit was developed while he was in HMP Shepton Mallet.
Harry Fletcher, Napo assistant general secretary, said: "The implications of the switch are very serious.
"An increase in heroin dependency means an increase in crime. Needle- sharing is a high-risk activity and drug abuse worsens control problems."Reuse content