One of England's biggest prisons has seen an "undeniable" increase in violence by inmates and use of force by officers, inspectors have found.
HMP Oakwood, near Wolverhampton, was also said to be risking its stability through drugs being easily available.
But inspectors said Oakwood was an "impressive institution" that had seen "sustained" improvement.
Security firm G4S, which runs the site, said it faced the "same challenges" with drugs and violence as other jails.
In his report, Peter Clarke, HM Chief Inspector of Prisons, described Oakwood as "a story of steady and sustained improvement after what was a testing start six years ago".
Inspectors found relatively few prisoners reported feeling unsafe, but the "level of assaults on staff was particularly high" and "it was undeniable that violence had increased".
Oakwood, which opened in the village of Featherstone in 2012, must now prepare a strategy for reducing violence and drug supply, HM Inspectorate of Prisons said.
Drug availability and use within the jail, which houses 2,071 male inmates, was described as the "main threat to [its] stability".
"Well over half of those prisoners surveyed suggested drugs were easily available in the prison" but "actions to reduce drug availability were beginning to be effective", the report said.
Overall, the inspection said the prison met 48 out of the 68 recommendations set for it in 2014.
Inspectors praised its leadership and said the report's findings were "tremendously encouraging".
Prison director John McLaughlin said: "At Oakwood we face the same challenges with violence and drugs as many other prisons. We have more work to do to tackle these issues and I'm encouraged by the inspector's findings that the measures we have taken so far are making a difference."
A G4S spokesman said more than 90% of prisoners spoken to said excessive force had not been used against them but staff had received extra training about when to use force, which had led to complaints going down.
Peter Dawson, director of the Prison Reform Trust, said there had been "significant improvements in a prison that had a very difficult beginning".