Almost one third of Scotland's prisoners are using drugs at the time of their release, according to figures issued yesterday.
The SNP said the figures, which were published in a parliamentary written answer by Cathy Jamieson, the Justice Minister, showed the need for the Executive to do more to tackle the causes of crime.
But the Scottish Prison Service defended its work in tackling drugs in prison, pointing out that four out of five prisoners tested positive for drug misuse when they arrived in jail.
Mrs Jamieson's reply to the SNP back-bencher, Stewart Stevenson, was provided for the minister by Tony Cameron, the prison service's chief executive.
He said testing for drug use in prison indicated that fewer than 20 per cent of prisoners tested positive, but liberation testing last December suggested about 30 per cent were drug users on release.
Kenny MacAskill, the SNP's justice spokesman, said: "These figures clearly show that prisons are not best equipped to treat the problem of drug addiction."
He said rehabilitation, rather than prison, was needed in cases where petty offending was caused by a drugs problem.
"It makes little sense to leave a prisoner's drug problem untreated," he said.
"This will merely lock the offender into a cycle of offending and addiction that will not be broken until the root cause of the criminal behaviour - the drug addiction - is addressed. These offenders should be rehabilitated, and it is clear from these figures that prisons are not always able to do so."
However, the Executive said the "vast majority" of prisoners had a drug problem when they entered prison. The prison service had a range of projects to deal with this, but the high level of addiction reflected a problem for society, it said.
A spokeswoman said: "There is no suggestion that drugs are tolerated in prison - far from it." She said measures now going through parliament in the Management of Offenders Bill would seek to join up anti-drug measures within prison and the wider community.
A spokeswoman for the prisons' service said: "We take the issue of drugs very seriously. Out of every five prisoners arriving at our doors, four are testing positive for some sort of drug misuse.
"That goes down to 30 per cent on release, and that means we are tackling 50 per cent. It is an improvement, but we take it very seriously and intend to get it down further."
The figures were released the same day the Executive launched a hard-hitting advertising campaign on the dangers of smoking heroin.
The Executive move follows a major study into the deaths of 317 Scots in 2003 which found that relatively simple steps may have saved some of those lives.
Hugh Henry, the deputy justice minister, said the TV adverts would challenge the false notion of "heroin chic" and the idea that a heroin habit could be managed, particularly if the drug was smoked rather than injected.
© The Scotsman