The report said prison staff needed more training over the issue

A report, Lost, Bullied and Trapped, by the Prison Reform Trust said many were being bullied, while prison staff were not trained to deal with them.

It estimates that 500 prisoners have learning difficulties, and a further 1,500 need extra support.

The trust said efforts to help were "generally ineffective".

This amounts to almost 30% of Scotland's prison population.

The report said many of the prisoners were cut out of rehabilitation courses because of their illnesses.

At worst it is downright cruel to incarcerate people who may find it hard to understand what's going on, who are more likely to be picked on or bullied by other prisoners Juliet Lyon Prison Reform Trust Full interview

A total of 79% of the prison staff questioned said it was "unlikely" or likely "only in a minority" of cases that those with learning difficulties coming into prison would have their conditions noted.

Nancy Loucks, the report's author, said swift action was needed to stop prison becoming a "double punishment" for these inmates.

"We now know that significant numbers of prisoners are not receiving the support they need to address their offending behaviour, turn away from crime and lead constructive lives," she said.

"This cannot continue.

"There are islands of good intentions and good practice within prisons but it is time to conduct a major review of how people with such disabilities and difficulties are treated to stop prison becoming a double punishment."

Courses unsuitable

According to the research, prisoners with learning difficulties also had difficulty understanding the court and prison system, so courses designed to progress sentences were often unsuitable for them.

The report said this meant they were "stuck in a maze with no exit and likely to spend longer in prison than others with similar sentences".

Juliet Lyon, director of the Prison Reform Trust, said efforts to help prisoners with learning difficulties remained "generally ineffective".

"At worst it is downright cruel to incarcerate people who may find it hard to understand what's going on, who are more likely to be picked on or bullied by other prisoners and who may leave prison less able to cope and more likely to re-offend," she said.

The trust will submit its report to former first minister Henry McLeish, who will chair a new independent prisons commission.