A new system of counting prison places will cloud the true picture of overcrowding in Scotland's jails, according to the prisons watchdog.
The prisons inspector noted a slight fall in overcrowding
Dr 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.
However, prison officials said design capacity would still be a factor when measuring overcrowding.
Dr McLellan said overcrowding was a "slightly less grim story than last year, but only slightly".
He also noted a fall in the number of prisoners who were still having to slop out.
In his report for 2004-2005 Dr McLellan, a former moderator of the General Assembly of the Church of Scotland, said the total number of inmates peaked at 6,999, compared with 7,074 in the previous year.
Design capacity in Scotland's prisons was unchanged, meaning peak overcrowding fell by 1%. However, this was still 13% above design capacity.
On the issue of measuring overcrowding, he said: "A new system of counting this year will make this simple measurement of overcrowding more difficult in future.
"Under new contract arrangements, prisons are contracted to take a certain number of prisoners.
"That number may be considerably higher than the number of places in the 'design capacity' of the prison, but it will become the number against which overcrowding will be measured."
He cited the example of Inverness Prison which has a design capacity for 108 prisoners.
The jail was contracted by the Scottish Prison Service to have 160 prisoners which meant it was contracted for 48% overcrowding.
Dr McLellan said: "By this way of measuring Inverness Prison would not be considered overcrowded until its prisoner number was more than 48% above the number for which the prison was designed."
A spokesman for the Scottish Prison Service said: "The SPS have put in place performance contracts for each of our prisons. These contracts recognise that some prisons regularly experience overcrowding.
"Where overcrowding levels in these prisons exceed the level identified for contractual purposes, the governors of these establishments are able to enter into discussions for additional resources.
"The SPS continue to calculate the level of overcrowding based on design, as well as contractual capacity."
The chief inspector said the number of inmates still having to slop out using bed pots and pans had fallen from 1,324 prisoners - 19% of the prison population - to 616 or 9%, over the year.
No prisoners sharing cells were slopping out and Dr McLellan noted that the prison service aimed to have the practice confined to two prisons, Peterhead and Polmont, by the end of the year.
Dr McLellan said: "Slopping out and overcrowding are the twin curses of Scotland's prisons."
The chief inspector voiced concern at the plight of women prisoners, whose numbers increased over the year and in the practice of holding children in prisons.
He said provision for women held in small numbers in some jails was "very limited" and the presence of any child in prison was a sign of failure in the efforts at early intervention to stop youngsters falling into lives of crime.