Scotland's record prison population will keep rising over the next decade and could top 9,000 by 2015, new figures are expected to reveal.
Taxpayers are currently paying about 28 million pounds a year to lock up the country's 7,000 prisoners, but if the prison service's own projections are borne out, costs will rise to nearer 40 million pounds in ten years' time.
Meanwhile, ministerial plans to scrap the current system of early release are threatening to send the prison population spiraling even higher.
The rising population, which penal reform campaigners say is "out of control", will heap further pressure on the country's already overcrowded jails, which are housing 1,500 more convicted criminals than they are designed for.
Scottish Prison Service projections, to be published this week, will show the current upward population trend is set to continue, with top-end predictions of about 9,000 behind bars by 2015. The projections are based on current trends and policy and do not take into account the impact of future changes in sentencing.
Cathy Jamieson, the justice minister, has admitted that ending early release would put another 4,000 people behind bars, if no further measures are introduced to keep the prison population in check.
Plans which could end the automatic release of long-term prisoners after they serve two-thirds of their sentence are to be put before the Executive by the end of the year.
Last night, campaigners and opposition politicians accused the Executive of failing the public by presiding over soaring prison numbers.
Juliet Lyon, director of the Prison Reform Trust, said: "With prison numbers rising out of control, there is little hope that overcrowded jails will be able to do much, if anything, to prevent the next victim. Reoffending rates are very high and prison staff often don't have the time to give to individuals to tackle that. Prisons have turned into little more than warehouses."
Kenny MacAskill, the SNP justice spokesman, said:
"We need to get the minor flotsam and jetsam of our society, who are in prison because of social problems out of jail, and keep prison for dangerous and serious offenders who need to be behind bars."
The Sentencing Commission is said to be drafting a report which will recommend that judges be asked to set prisoners a specific term in detention to be followed by a period of close supervision in the community.
If the prisoner fails to behave or re-offends during that period, under the proposals it is likely they would be sent back to jail to finish their term.
A Scottish Executive spokeswoman said: "While we cannot comment on speculation regarding unpublished statistics, the Scottish Prison Service is currently investing the equivalent of £1.5 million a week to improve prison conditions and increase places. Two new prisons are at the planning stage."