Four officers at Birmingham Prison have been injured by an inmate thought to have been armed with glass from a broken TV set.
G4S, which took over the running of the prison last year, said the officers were hurt on Wednesday.
The staff, all members of the prison's healthcare unit, were wounded in an altercation with a prisoner on remand, the firm said.
All four needed hospital treatment, two of them for serious cuts.
Brian Clarke, from the Prison Officers Association, said the attack happened as guards were dealing with a prisoner who was on "special conditions" and required three officers to look after him.
He said the number of staff allocated to the prisoner had been increased following another incident at the weekend.
Mr Clarke said that as staff opened the cell door, the prisoner surged forward, grabbed glass from a broken TV set outside the cell and attacked the officers with it.
He said one officer at the jail, known locally as Winson Green Prison, had sustained a deep cut to his arm and would need surgery.
He described it as an isolated case and said there was not a systemic problem at the prison.
In a statement, G4S said: "The safety and welfare of our staff and those in our care is our utmost priority, and we have launched an immediate investigation into the circumstances surrounding this attack.
"Our thoughts are with our staff at this time."
An investigation into the attack is under way.
The Victorian prison, which can cater for up to 1,450 adult male prisoners, became the first UK jail to be taken over by a private company in October 2011 when G4S secured a 15-year contract.
In June a report by the chief inspector of prisons found the privatised jail faced "significant strategic challenges".
Nick Hardwick said HMP Birmingham was "a cleaner, safer and more decent place" since it was privatised last October, but warned the prison needed to improve its training and drugs programmes.
Another report, by the Independent Monitoring Board for HMP Birmingham in January, praised prison staff for continuing to cope with problems including gang culture.
The attack came to light as it was announced G4S had lost its contract to run Wolds prison in East Yorkshire and failed to win a number of other private jail contracts.
Wolds will return to the public sector when G4S's contract ends in July.