Four staff have been injured in an altercation with an inmate at Birmingham Prison, the private firm which runs the jail has said.
The staff, all members of the prison's healthcare unit, were injured in an incident with a remand prisoner on Wednesday, G4S said.
Two staff suffered serious cuts and all four were taken to hospital for treatment.
A spokesman for the firm said: "G4S has confirmed that four members of staff were injured during an incident at the prison yesterday morning.
"The staff, all members of the prison's healthcare unit, were wounded in an altercation with a prisoner on remand, with two staff receiving serious lacerations.
"All staff were transferred to local hospitals for treatment. The prisoner has been contained."
He went on: "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."
News of the attack emerged as the firm was told it will lose its contract to run the Wolds prison in East Yorkshire from next year and was unsuccessful in its bid to run other jails.
It is understood the prisoner used broken glass in the attack.