Prisoners with learning difficulties are being bullied and get little help from prison staff, a report has said.
Many staff said they lacked training on learning difficulty issues
The Prison Reform Trust polled almost 200 staff in England and Wales, at least half of whom claimed inmates were victimised for their disability.
The report says up to 24,000 (30%) of the prison population have a learning disability, but staff are not given the training or resources to help them.
Trust director Juliet Lyon said the situation was "downright cruel".
The research also found that 80% said it was "unlikely" or "only in a minority" of cases that people with a learning disability coming into prison would be flagged up.
Some 70% of the staff polled felt they were not adequately trained to handle prisoners with such disabilities, nor did they have enough time or staff.
They also identified grave concerns for prisoners in understanding court proceedings and the prison system.
One recounted a conversation with a young prisoner who said 'no' when asked by a judge if he was remorseful.
"This young man had not heard that word before," said the staff member.
The former prisons minister and chairwoman of the research programme, Joyce Quin, said such a lack of support for prisoners could not continue.
"There are islands of good intentions and good practice within prisons but it is time for the government to conduct a major review of how people with such disabilities and difficulties are treated, to stop prison becoming a double punishment."
Ms Lyon, director of the Prison Reform Trust, said without the right help, these prisoners had less chance of turning away from crime.
"At best, work with this group of prisoners is ineffective.
"At worst, it's 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 reoffend."