document: Safety at HMP Birmingham is at risk as a result, they said, after the 12-hour disturbance involving hundreds of inmates last December.
  The riot had a "profound" effect on staff who were in shock but committed to moving on, inspectors said.
  G4S, which runs the jail, said the findings were a "fair assessment" of challenges it faced.
  February's inspection was carried out to check on how prisoners were being housed after 500 were moved after the riot and not to look at why it happened, chief inspector of prisons Peter Clarke said.
  The findings come as wings of the jail that were damaged in the riot have fully re-opened to inmates.
  The inspection followed an assessment by the Independent Monitoring Board last October that said an "urgent" solution was needed to stop prisoners taking psychoactive drugs.
  But, inspectors said illegal drugs were still widely available in February, with 52% of inmates saying said it was easy to get them - including psychoactive substances like Spice.
  This was up from 37% at the last inspection, in 2014, which said conditions had improved at the Category B jail.
  One in seven inmates also said they had developed a drug problem at the prison and nurses treated prisoners under the influence of those substances on 52 occasions in January.
  The use of mobile phones and drones to arrange and deliver drugs was "a significant threat".
  Violence had increased "substantially", much of it linked to the drug use, and some prisoners felt unsafe, inspectors said.
  The disturbance took much of the newer accommodation out of use, leaving more than 900 prisoners in older Victorian accommodation.
  Despite improvements to some internal communal areas, many of the showers, stairways and sluices were dirty, along with cramped cells, many having broken windows.
  However, security processes in the aftermath of the riot were "broadly proportionate".
  Inspectors praised many positive interactions between staff and prisoners and said healthcare was generally good and community rehabilitation projects were working better than in other jails.
  Prison director, Richard Stedman, said: "We are resolute in our determination to move on, make progress and not be defined by December's disorder and this week the prison returns to its full operational capacity."
  Michael Spurr, HM Prison and Probation Service chief executive, said "There remains more to do to provide purposeful activity and to tackle violence and illicit drug use, but the staff and the leadership team deserve credit for the commendable way they have responded to the challenges to date.
  Frances Crook, chief executive of the Howard League for Penal Reform, said the prison was in the headlines for "all the wrong reasons last year" and remains a hotbed of violence.
  Source: HM Inspectorate of Prisons
summary: Violence is on the rise at a riot-hit jail because of a high volume of illegal drugs inside, inspectors say.