A prison officer was left unconscious after he was "strangled" by an inmate, a union has said.
He was one of two officers assaulted in separate attacks at HMP Lindholme in Doncaster on Thursday night, the Prison Officers' Association (POA) said.
The other officer was "punched in the throat" and was recovering at home, POA chairman Mark Fairhurst said.
The Ministry of Justice (MoJ) confirmed a worker at the jail was attacked and an inmate had been segregated.
Following the attacks, staff refused to go back into the category C jail and staged a protest.
'Feared for safety'
The POA said one of the officers "was strangled to the point of unconsciousness".
"Despite the seriousness of these unprovoked assaults the prisoners had to be put back in their own cells as the segregation unit was full, emphasising the problems at Lindholme," it said.
"Staff at Lindholme genuinely feared for their safety this morning and refused to enter the prison until safety assurances and plans were put in place."
Officers went back to work following a meeting with the governor.
Full details of both attacks have not been released by the MoJ, but it confirmed an officer was found unconscious and treated at the prison before receiving hospital treatment.
An investigation had started, it added.
One of the two inmates involved in the attacks had been segregated after a space had become available in the morning, while the other still remained in his cell "awaiting segregation", Mr Fairhurst said.
He described HMP Lindholme as "one of the worst performing prisons", with up to 24 prison officers getting attacked at the jail on a daily basis.
The jail is one of 10 the MoJ had previously identified as having "acute problems" with high drug use, violence and building issues.
Packages containing the Spice drug and mini mobile phones were said to be regularly thrown over the fence.
A report by inspectors in February found "shocking" levels of drug use at the jail were due to a large number of prisoners connected to organised crime.
Last month, prison staff across England and Wales took part in widespread protests over "unprecedented violence" in jails.