Levels of violence at a Liverpool prison are "rising sharply" because of gang issues and the availability of drugs, an inspection has found.
The HM Inspectorate of Prisons report said "psychoactive substances" such as Spice and Black Mamba were a "significant factor" at HMP Altcourse.
The illegal substances are synthetic cannabinoids, which act like cannabis.
However, the report also said the prison was still providing inmates with good training, work and education.
The report, which detailed an unannounced inspection at the Fazakerley prison in June, stated although prisoners "said they felt safe and the prison seemed calm, levels of assaults against prisoners and staff, bullying incidents and fights were high and rising sharply".
It pointed to a problem with "gang issues and the availability of drugs, particularly new psychoactive substances such as Spice and Black Mamba", at the prison.
The report also said "little had been done to address the disproportionate number of young adults involved in violent incidents" and that there was "little support for victims" and a "failure to take prompt, firm action against perpetrators".
Inspectors also found the prison was "overcrowded and many cells designed for one or two held an additional prisoner".
Opened in 1997, HMP Altcourse is a private prison run by security company G4S and was built to hold up to 1,324 prisoners.
Chief Inspector of Prisons Nick Hardwick said the "urgent priority" was to reduce violence.
"The prison needs to ensure it does this without damaging its longstanding strengths of positive relationships and good purposeful activity, which are critical if prisoners are to leave with decent opportunities," he said.
G4S's managing director for custodial and detention services Jerry Petherick said the prison was "managing a challenging population which is increasingly prone to violence".
"Reducing violence is our top priority and action is being taken in line with the recommendations from the report.
"The exceptional relationship between staff and prisoners, as highlighted by the report, is a great strength and will support these plans."
Michael Spurr, the chief executive of the National Offender Management Service, said the organisation would "monitor progress closely to ensure that the necessary improvements are achieved".