Image caption The Conservatives want inmates to spend less time in their cells

About 80% of prisoners in Scotland are functionally illiterate, according to figures released under a freedom of information request.

The information was released by the Scottish Prison Service (SPS) after an approach by the Scottish Conservatives.

The Conservatives have called for all inmates to be given full-time work or training.

The SPS said it had introduced a more robust method of measuring literacy levels among prisoners.

The newly-released figures also suggested that about seven-in-ten inmates have problems with basic arithmetic.

They are gaining nothing from stewing in their cells and watching TV all day when they could be making a positive contribution Ruth Davidson, Scottish Conservative leader

A previous estimate made by the SPS in 2010 put illiteracy levels at 50%.

The official response from the SPS to the Conservatives said that "approximately 81 per cent of prisoners screened were assessed as lacking functional literacy and 71 per cent as lacking in functional numeracy".

Scottish Conservative 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."

Ms Davidson added: "They are gaining nothing from stewing in their cells and watching TV all day when they could be making a positive contribution."

The SPS has stressed that, because of a new system for measuring prisoners' reading, writing and counting skills, no comparison can be made with figures released in the past.

A spokesman said: "These figures bear no direct comparison and have to be 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.

"Significant investment is being made to address this deficit."