Dozens of prisoners have been released by mistake as violence in jails surged to a new high, official figures reveal.
Seventy-one convicted criminals or suspects were erroneously freed in 2016/17 - a rise of seven on the previous year and the highest number since current records started a decade ago.
It means inmates were let out due to blunders at a rate of more than one a week.
A raft of statistics released on Thursday by the Ministry of Justice also revealed another jump in assaults across the prisons estate in England and Wales.
There were 26,643 assaults in the year to March, including a record 7,159 attacks on staff - equivalent to nearly 20 every day.
The disclosure on mistaken releases sparked criticism from campaigners and politicians.
A prisoner is officially classed as having been freed in error if they are wrongly discharged from an establishment or court when they should have remained in custody.
Fifty-eight of the erroneous releases occurred from prison establishments, while 13 happened during escort or at courts.
Prisoners released in error are not considered to be unlawfully at large, according to an MoJ report setting out the figures.
It says: "They are not culpable and may be unaware that they have not completed their sentence or have outstanding warrants.
"Depending on the circumstances of the case, they may not be actively pursued for return to custody."
Examples of mistakes behind erroneous releases include misplaced warrants for imprisonment or remand, recall notices not being acted on, sentence miscalculations or discharging the wrong person on escort.
Liberal Democrat chief whip Alistair Carmichael said: "It is beyond belief that 71 potentially dangerous prisoners have been released by mistake."
Rachel Almeida, of the charity Victim Support, said: "Many victims will be shocked by these figures, especially if it involves perpetrators of serious crimes."
The MoJ said releases in error are extremely rare but it takes cases very seriously, working with police to recapture offenders.
In findings that will prompt fresh scrutiny of the state of jails in England and Wales, it was also revealed that:
- There were 15 escapes from jails or prisoner escorts in 2016/17;
- Self-harm in jails reached a record high of 40,414 incidents in the 12 months to March 2017, up 5,749 (17%) from the previous year;
- Serious assaults on staff have trebled since 2013, reaching 805 in 2016/17;
- Assaults in female prisons reached 1,023, the highest level for at least nine years;
- The performance of 10 jails was rated as being of "serious concern".
Campaigners and watchdogs have issued a catalogue of warnings about violence, drug use and overcrowding across the jail estate.
Earlier this month Peter Clarke, the Chief Inspector of Prisons, warned that staffing levels in many establishments are too low to maintain order and described the conditions some inmates are held in as "squalid, dirty and disgraceful".
Peter Dawson, director of the Prison Reform Trust, said: "These numbers confirm what the Chief Inspector of Prisons has described in graphic detail - that our prison system is nowhere near being safe for those who live and work within it."
Ministers have launched a recruitment drive to add 2,500 frontline officers and put in place new measures to tackle the availability of mobile phones and drugs in jails.
Justice Secretary David Lidington said improving safety and security in prisons was his top priority.
He said: "These figures reinforce how crucial it is that we make progress as quickly as possible.
"I have seen first-hand the challenges our dedicated and hardworking prison staff face.
"Boosting the frontline is critical to achieving safety and the number of prison officers we are recruiting is rising, with the number of new prison officers joining the service at its highest level since 2010."