About 1,800 inmates are taking literacy classes each month
Half of all male inmates attending prison education classes in Scotland are illiterate, new figures suggest.
Scottish Labour said the statistics were "shocking" and called for immediate action to tackle the problem.
The Scottish Prison Service also revealed that 1,800 inmates are attending classes to learn to read and write each month.
Education Secretary Mike Russell recently announced that the government was to compile a literacy action plan.
A Scottish government spokesman said: "We entirely agree that improving reading, writing and numeracy skills improve self-esteem and employability skills, which can reduce the likelihood of prisoners reoffending.
"However the numbers of prisoners serving short sentences means that assessment and matching needs is not always easy."
Labour's justice spokesman Richard Baker said it was vital that changes be made to the education system in order to eradicate illiteracy.
He added that it would not only improve the chances for all but would boost business and reduce the number of individuals getting involved with crime.
Labour's Literacy Commission found last year that one in five children leaves primary school unable to read and write.
Mr Baker said the commission's report provided a "roadmap for success" and called for it to be implemented.
The Scottish Prison Service said that about 50% of men attending prison learning centres are "functionally illiterate", below SQA intermediate level 1.
The government insisted there were examples of good practice in the country's prisons.
A government spokesman said: "These include prisoners helping each other and programmes which, for example, can help prisoners communicate better with their families."