Prisoner numbers in Scotland's jails could increase by about a third in the next 10 years, according to official predictions.
Ministers' policies could halt the rise in the jail population
However, managers of the Scottish Prison Service said ministers' policies could affect future trends.
They said a range of alternatives to custody being applied by the courts might halt the predicted 37% rise.
Figures suggested up to 9,300 people could be behind bars in 2015, compared to about 6,779 at present.
However, a report on past trends which was compiled under the assumption there would be no change in sentencing policy, said there could be a peak of 7,900 or the number might fall to 6,600.
Two new prisons have been planned to increase capacity by about 1,100.
In addition to the number of people being sent to prison in the future being affected by alternatives to custody, a trend towards longer sentences could also affect the statistics.
'Build more jails'
Long-term inmates account for almost half of the population.
Latest figures were based on past trends and assumed there would be no change in sentencing behaviour by the courts.
Scotland's prison population reached its highest recorded level - 7,072 - in March 2004.
If prisoner numbers continue to rise, more ways of rehabilitating offenders in the community would have to be developed - as campaigners prefer - or more jails built.
The prisons watchdog said earlier this year that a new system of counting prison places would cloud the true picture of overcrowding.
Chief prisons inspector Andrew McLellan pointed to changes which mean the number of prisoners contracted to a jail might far exceed its design capacity.
This could mask the real extent of overcrowding, his annual report said.