Prisoners at HMP Ranby in Nottinghamshire – dubbed the prison with the worst conditions in the whole country – took over a wing of the prison at 1pm today.
The Prison Officer's Association (POA) has told IBTimes UK that there is an ongoing "serious disturbance" at the prison as 120 prisoners have "taken control" of a part of the prison where cell accommodation is located.
At least one fire has been started and dealt with inside the prison since the incident began, Glyn Travis from the POA said.
At 4.15pm on Saturday specialist prison guards, similar to riot police who deal with disturbances in prisons, were deployed.
Staff on weekend leave have also been called back to work to help negotiate with prisoners during the stand-off.
It is believed the incident started when prisoners were denied an afternoon activity that meant they could use the prison's outside spaces.
"The weather is very hot down at Ranby, the prisoners are at breaking point. 120 men thought they would be going outside today, but it was cancelled, problems start to snowball and now there is a big problem," said Travis.
"We will find out this afternoon how many prisoners are willing to surrender peacefully and how we will get the situation under control. There are around 60 prisoners who have been very active in demonstrating," Travis continued.
"It could take all night to resolve."
The POA attributes the disturbance to a build-up of poor conditions and "shortfalls" in the performance of staff at the prison, which were outlined in the report by Her Majesty's Inspectorate for Prisons (HMIP) last week following an unannounced inspection.
The HMIP found that Ranby, which holds more than 1,000 men, was dirty and nearly half the population said they felt unsafe having been victimised or intimidated.
Prisoners have been known to climb netting designed to prevent suicides in a display of bad behaviour that will force their transfer to the segregation unit of the prison where they "feel more safe", the report stated.
Travis told IBTimes UK: "Every incident you can imagine that happens in a prison has doubled in occurrence in the last year at HMP Ranby.
"Last week's report is a mirror of what is happening now at the prison. The disturbance is the conditions being shown in effect."
According to the HMIP during the last 12 months there have been 534 reported incidents inside the Ranby prison, including 133 assaults and 144 acts of self-harm.
These statistics have been branded "ridiculous" by the POA, which noted that there were just 280 reported incidents at the prison between 2012 and 2013.
There are worries that prisoners will damage cell accommodation during today's incident, which could lead to major problems for an institution that is already at maximum capacity, Travis said.
"If accommodation is damaged we'll have to start moving prisoners which will cause even more problems."
A spokesperson for Her Majesty's Prison Service 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."