A prison officer was left unconscious after being attacked by an inmate.
The officer was assaulted overnight at HMP Lindholme, near Doncaster, and later went to hospital, the Ministry of Justice (MoJ) confirmed.
Prison staff initially refused to go back on to the wings following the attack and staged a protest in the visitors centre, BBC home affairs correspondent Danny Shaw said.
The prisoner involved had been segregated, an MoJ spokesman said.
Full details of the attack are not yet known but the MoJ confirmed the officer was found unconscious and treated at the prison before receiving hospital treatment.
An investigation had started, it added.
Staff at the Category C jail went back to work following the protest and a meeting with the governor over safety concerns.
HMP Lindholme is one of 10 jails the MoJ previously said had “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.