Six prison officers were injured after disorder erupted at a high-security jail holding some of the country's most dangerous inmates.
One member of staff suffered a suspected broken jaw during the disturbance at Category A HMP Long Lartin on Sunday, Prison Officers' Association (POA) spokesman said.
It is understood the disorder was confined to one wing of the Worcestershire jail and began at around 9.30am before being brought under control by around 6pm.
Seven prisoners have been placed in isolation to be transferred to other prisons.
Shadow Justice Secretary Richard Burgon tweeted: "It is unacceptable that prison staff face such extreme violence at work.
"The government must immediately get a grip on the chaos in our prisons."
A Prison Service spokeswoman said: "We do not tolerate violence in our prisons, and are clear that those responsible will be referred to the police and could spend longer behind bars."
An investigation is under way.
Long Lartin holds some 510 prisoners with more than three-quarters of inmates serving life sentences, according to a HM Chief Inspector of Prisons report this year.
Opened in 1971, it has housed a number of high-profile prisoners, including radical Muslim cleric Abu Hamza and murderer Christopher Halliwell.