A serious disturbance has broken out at a “crisis” prison after 120 inmates refused to return to their cells this afternoon.
The Prison Officers' Association (POA) said a fire was started during the incident at HMP Ranby in Nottinghamshire.
The disturbance is ongoing.
A spokesman for the Prison Service said minor damage was sustained and there were no injuries to staff or prisoners.
Chief Inspector of Prisons Nick Hardwick said HMP Ranby was “in crisis” after the publication of a report on Thursday which described the prison as unsafe with high levels of violence.
Glyn Travis, assistant secretary of the POA, said: “There is a serious incident at HMP Ranby.
“At around 12.30pm 120 prisoners refused to return to their cells. They have taken control of a unit.
“We have got national resources at the prison trying to establish a surrender plan.
“We know 60 prisoners have been actively involved.”
He said the inmates started a fire which was quickly put under control.
A Prison Service spokesman said: “A disturbance involving between 30 and 60 prisoners at HMP Ranby started on one wing at around midday on 26 July.
“Minor damage has been sustained and there have been no injuries to staff or prisoners.
“Prison staff are being deployed to resolve the incident safely.”
A critical report on HMP Ranby was released following an 11-day inspection by HM Inspectorate of Prisons.
It found that two prisoners died through “self-inflicted deaths” last year, and a further two have died in a similar way since the unannounced inspection in March.
Conditions in part of the prison, which holds more than 1,000 men, were dirty, prisoners were found to have climbed netting in a bid to force a transfer to another facility and nearly half the population said they had felt unsafe having been victimised or intimidated, findings showed.
Evidence was found of an increasing number of incidences of self-harm at the category C training prison, and the availability of legal highs was also found to have increased.
Labour MP John Mann, whose Bassetlaw constituency includes the prison, said his repeated warnings that “dysfunctional” management and serious staff shortages would lead to disaster had been ignored by ministers and prison officials.
He said there were 80 fewer prison officers than previously - a 20 per cent reduction - with many experienced individuals having been “forced out” over the past two years.
“It is a prison where, for quite a time, it's been clear prisoners have been running the prison,” he said.
“There are not the numbers or the expertise among the staff to deal with it.
“The governors were warned, the Government was warned, by me and by many others, that this would lead to disaster.
“I have raised this directly with the prisons minister more than once.”
He welcomed the appointment of a new governor but said he had “lost track” of how many changes there had been at the top and insisted the only solution was to recruit more staff.
Mr Travis said today's disturbance was “no surprise” to the POA.
“It's all down to chronic staff shortfalls and a management that are hell-bent on delivering things they can't deliver safely,” he said.
“Prisoners are saying 'We're not dealing with this'.
“They will react.”
There was little sign of the disturbance outside the main entrance to the prison, which is on a country road a few miles outside Retford.
Prisoner-carrying vehicles from the company GEOamey were parked in the main car park along with an ambulance and a number of vehicles from Nottinghamshire Police's Operational Support Group.
Officers carrying shields and other equipment were seen going into the prison earlier.
Shortly after 7.30pm around 15 firefighters turned up outside and began putting on their full kit before walking into the prison through the main gate.