Ten prison staff were injured after being attacked by as many as a dozen inmates, the Prison Officers Association (POA) has revealed.
The guards were attacked at the category B Swaleside jail on the Isle of Sheppey in Kent on Thursday night after challenging a prisoner whom they believed was in possession of a mobile phone.
The inmates joined in the violence as the prisoner was being escorted to his cell, the POA said.
The incident came after rampaging inmates torched buildings and smashed windows at Ford open prison near Arundel, West Sussex, on New Year's Day.
Only staff bravery prevented a similar riot at Swaleside, the POA said.
Colin Moses, the organisation's national chairman, said: "This again highlights the level of violence faced by our members on a daily basis in our dangerously under-resourced prisons".
One member of staff was taken to hospital for treatment for concussion and nine others suffered minor injuries or bruising, the POA said.
"The incident took place after staff challenged a prisoner who was believed to be in possession of a mobile phone," the POA said.
"As the prisoner was being taken back to his cell, staff were attacked by 10 to 12 other prisoners. At the time of the incident 126 Cat B prisoners were unlocked on association and some of those prisoners joined in the incident. Staff raised the alarm and were swiftly joined by colleagues who then assisted in restoring order to the wing.
"It is through the bravery of staff at HMP Swaleside that another incident on the scale of the HMP Ford and HMP Moorland riots were averted."
A Prison Service spokesman said: eight prisoners were involved in "an incident of concerted indiscipline".
"The incident was quickly resolved by staff and concluded at 7.35pm. Several members of staff have been treated for minor injuries. This incident will be subject to an investigation and the appropriate disciplinary action will be taken," he added.