Glyn Travis, assistant secretary of the Prison Officers' Association (POA) 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. Prisoners are saying 'We're not dealing with this'. "They will react."
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% 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. 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."
"According to Ministry of Justice documents, Ranby can hold 892 prisoners in safe and decent accommodation. It holds 1,088."
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.
Chief Inspector of Prisons Nick Hardwick said the report showed a prison "in crisis".
Glyn Travis, assistant secretary of the Prison Officers' Association (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."
A Prison Service spokesman said: "A disturbance involving between 30 and 60 prisoners at HMP Ranby started on one wing at around midday. 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 disturbance has broken out at a prison after 120 inmates refused to return to their cells this afternoon. The Prison Officers' Association confirmed the "serious incident" at HMP Ranby in Nottinghamshire started at around 12.30pm and is ongoing.
A spokesman for the Prison Service said minor damage was sustained and there were no injuries to staff or prisoners. On Thursday a report described HMP Ranby as unsafe with high levels of violence.
Police in South Yorkshire are searching for a man who escaped from a prison in Doncaster.
24-year-old Damian Pearson absconded from HMP Hatfield at around 7am this morning. He was sentenced to three-and-a-half years in prison for burglary at the beginning of last year.
Police in South Yorkshire are warning the public not to approach two men who absconded from prison in Doncaster.
Damien Burns, 39, originally from Scarborough, North Yorkshire, and Dean Jackson, 27, from Newcastle-upon-Tyne, absconded from HMP & YOI Hatfield on Monday evening.
The pair are not thought to have any ties in South Yorkshire so officers believe they may be trying to make their way back to their home towns.
Burns was serving an indeterminate sentence, since 2007, for a knifepoint robbery and Jackson was on remand awaiting a sentence for a theft related matter.
Anyone who sees either of the men is asked not to approach them and to contact police immediately.
An on-the-run prisoner serving a life sentence for killing his girlfriend in Lincolnshire has been located and returned to jail. Christopher Woodward absconded from HMP Prescoed on 17 January. He was found guilty of killing Jo-ann Noble and leaving her body in bed for nine days in 2000.
Gwent Police say he was located in the South Wales Police area in the early hours of this morning
A 73-year-old prisoner - serving a nine-month sentence for voyeurism - died in prison from a serious head trauma, police have said.
Alan Goode was found in his cell by staff at Lincoln Prison in the early hours of yesterday morning.
Good had been jailed after being found on the floor of a cubicle in the women's toilets at a Leicestershire shopping centre.
A 35-year-old man - understood to have been sharing a cell with him - was arrested in connection with the incident but has since been bailed for psychological assessment.