Uncertainty and confusion over the future of an already overcrowded jail has thwarted some of the improvements demanded by prison inspectors.
The prison inspectorate said High Down, in Surrey, fared worse in two "healthy prison" tests than it did in the previous inspection three years ago.
Chief inspector Peter Clarke said violence had also increased, with much of the problem related to drugs.
High Down said new staffing would allow it to make the required improvements.
Mr Clarke said only two of 14 recommendations relating to "purposeful activity" - an area of prison life that includes training and education - had been implemented since its previous inspection in 2015.
It is now ranked "poor" in that category - the lowest possible.
High Down's grade for prison safety had also declined since the last visit, with violence now at similar levels to that at other "local" jails.
The report said that of the 1,130 men held there, 400 were in overcrowded cells.
Mr Clarke said there was a shortfall of about 550 activity places at the time of his team's visit and attendance was only around 55%.
He said there appeared to be confusion over when plans to convert the category B jail to category C - scheduled for this autumn - would happen, and there was no evidence of concrete preparations.
He had been told HM Prison and Probation Service (HMPPS) had not kept management updated - a situation he described as "extraordinary".
In his report Mr Clarke said: "The current leadership and staff of the prison are clearly committed to doing what they can for the men in their care... In turn, the prison itself needs and deserves practical support from HMPPS."
Michael Spurr, chief executive of the prison service, said: "High Down has received a significant influx of new staff which will enable the governor to achieve sustained performance improvement over the coming months."
He said the change to a training prison would not take place "until we are satisfied that the prison has the resources required to effectively fulfil its new role".