Prisons in England and Wales have become "unacceptably violent and dangerous places", the chief inspector of prisons has warned.
In his first annual report, Peter Clarke said the "grim situation" revealed in last year's report was now "even worse" is some areas.
Reforms have been jeopardised by legal highs among inmates, he added.
New Justice Secretary Liz Truss said she was under "no illusions" about the scale of the challenge to bring reform.
Last year, the outgoing inspector said prisons were in their worst state for 10 years.
Mr Clarke - who was appointed as chief inspector of prisons in January - said in some key areas the situation had got even worse since then.
He said there was "a simple and unpalatable truth about far too many of our prisons", adding: "They have become unacceptably violent and dangerous places".
Last year, there were more than 20,000 assaults - 27% up on the previous year - with six homicides in the 12 months to April 2016.
Incidents of self-harm rose by a quarter in 2015 to more than 32,000.
Mr Clarke said a large part of the violence was linked to the harm caused by new psychoactive substances, often referred to as legal highs, which were having a "dramatic and destabilising effect".
The Prisons and Probation Ombudsman identified 39 prison deaths linked to the use of legal highs between June 2013 and June 2015.
However, Mr Clarke said there was still "no national strategy" for dealing with the problem of legal highs in prisons.
He said he had met prisoners who had "self-segregated" in order to escape the violence caused by legal highs.
While some aspects of the problem with legal highs were being addressed by criminalising possession and better use of technology to test for the drugs, it wasn't enough, he added.
"A very clear strategy is required to deal with this threat to our prisons," he said.
Ms Truss said she wanted to see "radical reform" but was under "no illusions about the scale of the challenge we face or how long reform takes".
She was appointed as the justice minister last week - replacing Michael Gove, who had announced a series of reforms for prisons.
She said: "We have an ambitious agenda to modernise the prison estate, improve education and empower governors, so that we can tackle issues like drugs and violence which are key to cutting reoffending and keeping staff and prisoners safe."
Ms Truss said the report showed "why prison reform is so important", adding that she would "set out the next steps for this agenda in coming weeks".