At least four out of five prisoners are functionally illiterate, according to the Scottish Prison Service (SPS).
A total of 5,575 prisoners opted to be assessed for literacy and numeracy using a screening programme between August last year and July 31.
Around 81% lacked functional literacy while 71% lacked functional numeracy, said the SPS.
Scotland's prison population stands at around 8,000.
A person who is functionally illiterate is broadly defined as being unable to read and write well enough to deal with everyday life requirements.
The Conservatives said the figures are much higher than a previous statistic quoted by the prisons service which, in 2010, said 50% of prisoners learning to read and write were functionally illiterate.
Tory leader Ruth Davidson said: "These findings show just how acute the problem is with prisoners lacking basic skills in maths and English. That severely hampers their chances of securing employment when they are released, not to mention the fact they also have a jail term under their belt.
"This is why we need to introduce full-time work and training for prisoners as soon as possible. They are gaining nothing from stewing in their cells and watching TV all day when they could be making a positive contribution."
A spokesman for the prison service said the latest figures bear no direct comparison with the 2010 statistic and have to been seen in context.
"Since 2011 we have introduced a more robust method of assessing the literacy and numeracy levels of those people entering our prisons," he said. "Significant investment is being made to address this deficit."