Calls for urgent action to combat "disarray" in Britain's prisons are mounting after a second day of disorder at The Mount in Hertfordshire.
Riot squads were sent into the jail, in Bovingdon in Hertfordshire, after prisoners armed with weapons reportedly took over and vandalised its 250-inmate Nash Wing.
A spokesperson for the Ministry of Justice (MoJ) said order had been restored by around 5pm.
“Specially trained prison staff have successfully resolved an incident at HMP The Mount on 1 August. There were no injuries to staff or prisoners," she added.
“We do not tolerate violence in our prisons, and are clear that those responsible will be referred to the police and could spend longer behind bars.”
An earlier statement said that the violence had been "contained" and there was no risk to the public.
The disturbance came after authorities quelled violence in two wings the previous day, and separate incident broke out at HMP Erlestoke in Wiltshire, where inmates escaped their cells and rioted in two wings in June 2016.
“Staff successfully resolved an incident involving a small number of prisoners at HMP Erlestoke on 1 August,” a Prison Service spokesperson said.
“The offenders responsible will be referred to the police and could spend longer behind bars.”
Both jails house “category C” male prisoners, who are considered too dangerous for an open prison but are deemed unlikely to attempt escape.
Unrest broke out at The Mount on the same day a report warned of the threat of violence following a series of incidents last summer, saying experienced staff had been lost.
The "Tornado" riot squad was sent in while police vehicles were pictured at the prison and ambulance crews and fire engines massed at a former airfield nearby.
A spokesperson for the Prison Service said the incident was over by around 10pm, adding: “Specialist prison staff resolved an incident involving a number of prisoners at HMP The Mount on Monday evening. There were no injuries to staff or prisoners.
"The offenders responsible will be referred to the police and could spend longer behind bars."
A spokesperson for Hertfordshire Constabulary spokeswoman said it was working with the MoJ to review what, if any, criminal offences had been committed.
Amid increasing concern over the state of British prisons and the impact of Government cuts and privatisation, one commentator had described The Mount as ”on the brink of a meltdown“.
Recent weeks have seen some inmates locked in their cells all day, with cold food delivered to their cell doors, prison affairs academic and blogger Alex Cavendish said.
A woman who said her son is an inmate tweeted that he had been on “24 hour lock down for weeks” at The Mount, which houses more than 1,000 inmates, while others had complained of not being allowed out to shower or make phone calls.
Prison workers’ union the POA said Monday’s disorder exposed a “service in crisis”, reported dozens of officer vacancies.
“The incident involved more than 200 prisoners at some points and it took more than six hours to resolve the situation,” a spokesperson said.
Mark Fairhurst, acting national chairman of the POA, said the recent unrest underlines “disarray” in British prisons, which he said had lost 8,000 staff since 2010 despite inmate numbers increasing.
“Morale is at an all-time low and as fast as the Service recruit new staff, staff leave in their droves,” he added, calling for an independent review and the 1 per cent pay restraint being scrapped.
“It is time for the Government to act and act quickly.”
An assessment from the jail's Independent Monitoring Board (IMB) said that last summer “all the ingredients were in place for The Mount to suffer disorder such as has been experienced in other prisons - staff shortages, readily available drugs, mounting violence”.
The report, covering the 12 months to the end of February, said that during the year The Mount has struggled with staff shortages “driven by uncompetitive pay scales”.
It added: “Experienced staff have left and not been fully replaced, so that at the end of February there were 24 vacancies out of a complement of 136 officers, and a high proportion of officers and managers had less than two years' experience.”
Despite the shortages the IMB said the establishment ended the year as a safe prison where prisoners have a good chance of rehabilitation, with the governor and staff managing to control violence.
It said the drug problem at the jail was most acute in November when a number of prisoners suffered serious short-term health problems with 70 emergency call-outs in the month, mainly for drug-related problems.
The synthetic cannabis substitute known as Spice is a “big concern”, the report said, adding that while drone deliveries have declined, “the supply is still getting in”.
Statistics released by the MoJ last week showed that attacks and self-harm have reached an all-time high in prisons in England and Wales, where there were 97 suicides in the past year.
There were 26,643 assaults in the 12 months to March, including a record 7,159 attacks on staff - averaging out at nearly 20 every day.
Campaigners and watchdogs have issued a catalogue of warnings about violence, drug use and overcrowding across the prisons estate in England and Wales, which has been hit by a number of disturbances in the last year.
The Howard League for Penal Reform said chronic overcrowding and staff shortages were driving a “rising tide of violence, human misery [and] chaos”.
“How many people have to die before action is taken?” asked chief executive Frances Crook.
“The first step to recovery is to recognise that there is a problem. The second step is to do something about the problem.
“By taking bold but sensible steps to reduce the prison population, we can save lives and prevent more people being swept away into deeper currents of crime and despair.”
Last month Peter Clarke, the Chief Inspector of Prisons, warned 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”.
Additional reporting by PA