An open prison is in danger of becoming a human "warehouse", a report by the chief inspector of prisons has warned.
The new system has created 27 extra prisoner spaces at Noranside
A new system at Noranside jail which allows prisoners to spend one week each month on home leave has freed up space.
However, Dr Andrew McLellan said it was having a major impact on the running of the prison in Angus, particularly on Wednesdays, the "changeover day".
He raised "serious concerns" that work and education opportunities had been sidelined to make the new system work.
Dr McLellan said: "Noranside needs to be careful that the prison does not become a 'warehouse' for prisoners between home leaves.
"Significant parts of the regime are currently being sacrificed for the new system of 'continuous cell occupancy' and extended home leave to be managed.
"This concern was raised forcefully during the inspection by the visiting committee."
The inspector sounded the warning in a report on Scotland's two open prisons, Noranside, near Forfar, and Castle Huntly, near Dundee.
The introduction of continuous cell occupancy at Noranside has provided 27 extra prisoner spaces.
"These arrangements have been managed very well, but the impact on the rest of the regime is significant and needs to be monitored," Dr McLellan said.
He said continuous cell occupancy, and the opening of new accommodation which has doubled the size of Castle Huntly, had increased the numbers in open prisons to 168.
This increase in numbers posed "a real challenge", he warned.
"While the amount of work has increased at Castle Huntly, it is important that work places increase to meet the new numbers.
"The same is true of education," he said.
Overall, the report found there had been major improvements in conditions in the open prisons.
It said accommodation had improved and prisoners were spending less time locked in their rooms at Castle Huntly.
"This inspection shows that considerable attention has been paid to the recommendations made in my last full inspection report published in 2005," Dr McLellan said.
"That report was critical of several aspects of the open estate, many of which have now been addressed."