Staff at a private prison where disorder broke out predicted there would be trouble when the workforce was dramatically cut, a union official has said.
Around 50 inmates at HMP Northumberland were involved in the serious disorder on Friday night.
It required the assistance of specially-trained colleagues from other prisons around the North East to stop the trouble around seven hours later.
Terry Fullerton, who represents the region on the Prison Officers' Association national executive committee, said members had raised concerns about staffing levels as far back as December.
He said since then around 130 staff had left, leaving fewer than 200 uniformed staff to guard 1,350 inmates.
"Staff were concerned that the reduction would lead to something like what came to happen on Friday night," he said.
Inmates refused to return to their cells and warned staff to leave the area, which they did, allowing them to take over until a specialist squad of officers was assembled.
On-site services provider Sodexo said an investigation was being held into what happened.
Mr Fullerton believed inmates will have come to realise staffing levels were low enough for them to take over.
He said: "It doesn't take inmates long to realise that staffing levels have reduced, and that there are less of the 'white shirts' that are needed to keep control."
The union official said inmates at HMP Northumberland had been risk-assessed and deemed to be suitable for a category C prison, yet some of them would be convicted murderers, paedophiles and drug dealers.