A record 71 prisoners were released from jails in England and Wales last year by mistake, figures have shown.
Data from the Ministry of Justice (MoJ) also showed a rise in violence with 26,643 assaults in the year to March 2017 - 20% more than the previous year.
Of these, a record 7,159 were attacks on staff - equivalent to 20 every day.
Justice Secretary David Lidington said frontline staff were critical to achieving safety and the number of prison officers recruited was rising.
There were 15 escapes from prisons in 2016-17 - up by two on the year before - and the number of prisoners released by mistake rose by seven to 71 - the highest number in any year since current records began in 2006-07.
Of those released in error, 58 occurred from prison establishments while 13 were during escort or at courts.
Those released by mistake are not considered to be unlawfully at large, according to the MoJ report setting out the figures.
It said: "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."
The figures show there were 97 self-inflicted deaths in the year to June 2017 - down 10 on the year before.
Overall, the performance of prisons appears to have worsened, with the MoJ naming 10 in England and Wales as causing "serious concern".
The 10 jails are Bedford, Birmingham, Bristol, Brixton, Guys Marsh, Hindley, Liverpool, Pentonville, Wandsworth and Wormwood Scrubs.
Last year six prisons were assessed as giving serious concern.
A further 40 prisons have been ranked as causing concern, while nine have been graded exceptional.
'Nowhere near safe'
The justice secretary 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."
But Peter Dawson, director of the Prison Reform Trust, said the issues raised require more than the promise of new staff.
He 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.
"In the short term, the provision of much cheaper and easier access to a legitimate phone system would make a day to day difference - and provide some consolation to the families of prisoners wondering if their loved ones are safe inside."
The union representing prison workers has called for the resignation of prisons' boss Michael Spurr.
The general secretary of the Prison Officers Association, Steve Gillan, said his members had "lost patience".
"They are fed up with being treated with disrespect, ignored and used as modern-day slaves as the management of HMPPS (Her Majesty's Prisons and Probation Service) try to paper over the cracks," he said.
Those "at the top" had been rewarded with bonuses which outweigh the salary of some members, he added.
Earlier this week it emerged that Mr Spurr had received a bonus of up to £20,000 in 2016-17 on top of his annual salary of around £150,000.