Fears over the planned merger of two jails in Suffolk have been expressed by the Independent Monitoring Board (IMB), which reports on prison conditions.
The IMB's annual report for HMP Edmunds Hill, Stradishall, said it had serious reservations about the proposals.
The prison service wants to cut costs by merging staff at the 370-inmate jail with HMP Highpoint which houses 1,000 prisoners across the road.
The IMB said the merger could impact on prisoner training and rehabilitation.
It said it was also worried about its future as an effective monitoring service.
The chairwoman of the IMB, Dorothy Higginson said: "In this particular case while they are both Category C prisons they have completely different purposes.
"We have a lot of prisoners at the end of their sentences who need thorough rehabilitation and training in readiness for release."
Mrs Higginson said Highpoint inmates had more long-term prisoners and lifers and their training needs were very different to those at Edmunds Hill.
A spokesperson for the National Offender Management Service said: "We aim to get good value for money for the taxpayer by meeting prison capacity requirements more efficiently.
"We are discussing proposals to cluster HMP High Point and HMP Edmunds Hill. No decisions have yet been taken, but we are keeping staff updated on the process."
The full extent of the plans which could save up to £500,000 are expected to be revealed in July or August and will be implemented next April.