At least four out of five prisoners are functionally illiterate, according to figures from the Scottish Prison Service.
A total of 5575 prisoners were assessed for literacy and numeracy. Around 81 per cent lacked functional literacy, while 71 per cent lacked functional numeracy.
A person who is functionally illiterate is defined as being unable to read and write well enough to deal with everyday life requirements.
Scottish Tory leader Ruth Davidson said the problem was worse than previousy admitted and called for full-time work and training for prisoners.
The SPS said significant investment was being made to address the issue.