The merger of two prisons in East Yorkshire has been "traumatic and prolonged", inspectors have found.
HMP Humber was created when a secure corridor was built last year to connect Wolds and Everthorpe prisons.
A report found the amalgamation took far longer than expected. The prison was also deemed unsafe with a high number of assaults on staff.
Chief inspector of prisons Nick Hardwick said the change "was a significant factor" in the failings.
The HM Inspectorate of Prisons report revealed "significant" delays in the corridor being built and changes to the staffing structure.
"During this period of uncertainty many aspects of the running of the two sites had deteriorated", it stated.
The report detailed how staff were dissatisfied with the "management of the merger" and differences in pay, as well as "cultural differences" between the two sites.
Inspectors also noted poor behaviour by prisoners was commonplace and there were "failures" in the adequate treatment of inmates and in safety procedures to protect them at the category C jail, which holds 1,002 convicts.
Mr Hardwick said: "Humber was a prison undergoing major change. The merger had been traumatic and prolonged, and some aspects of the management of the process could have been better.
"Managing the introduction of new providers in health, substance misuse and resettlement added considerably to an already complex picture."
Michael Spurr, chief executive of the National Offender Management Service, said: "The governor and staff at Humber have managed significant change following the merger of two prisons and are now starting to make real progress in establishing an effective regime.
"This includes a comprehensive plan to improve safety, over 100 extra work and training places and a new abstinence and recovery wing to help prisoners with substance misuse issues.
"The chief inspector recognised the prison's commitment to improve and staff will be fully supported to do so."
Wolds prison was previously run by private security firm G4S before its contract ended in 2013.
Wolds joined with Everthorpe in April last year and the corridor linking the two sites opened 13 months later.
The Ministry of Justice said the merger "allowed us make the most of the resources available while still providing a safe and decent regime".
"We have a zero tolerance approach to violence in prison and work closely with the police to prosecute those who assault our staff or other prisoners," a spokesman added.