Restricting legal aid for prisoners could lead to their rights being violated, says Caritas Social Action Network (CSAN).
The Ministry of Justice is currently consulting on whether to remove legal aid from all treatment cases.
Treatment issues encompass prisoner concerns about discrimination, communications, mother and baby issues, and concerns about the behaviour of staff, such as bullying.
The current system allows prisoners to apply for advice and assistance funded by criminal legal aid on matters relating to treatment issues that are not suitable to be resolved through the internal prisoner complaints system.
The Government expects to save £4million a year by cutting legal for prisoners.
However, CSAN is advising the Government not to use this as a way of making savings, saying it is not "just".
CSAN's Public Affairs Officer Liam Allmark said: "As a society we have a responsibility to ensure the welfare of prisoners. Whilst they are properly deprived of their liberty, they should never be mistreated or denied recourse to justice where mistreatment occurs."
"Strict criteria already exist for prisoners accessing legal aid in treatment cases. Withdrawing legal aid altogether will remove safeguards and increase the potential for prisoners' rights to be violated.
"Whilst we recognise that difficult decisions have to be made in a time of austerity, cutting such a vital recourse to justice for some of the most vulnerable members of our society is neither a just nor an equitable way to make savings."
CSAN is the social arm of the Catholic Church, comprising of 41 member charities, who advocate on a range of different issues including prisoners' rights.