A prison officer has been arrested and seven others suspended over allegations they goaded Muslim inmates until they needed to be restrained and "kept score" in a group text message.
The officers at HMP Nottingham are alleged to have targeted mostly black and Asian prisoners, but also extended the abuse to white converts to Islam and vulnerable drug addicts.
They are accused of having placed bets and tallied up points on the messaging service WhatsApp, with the highest score given to those who restrained converts.
"The officers are said to have engineered a confrontation so they could have reason to restrain inmates. The language they are said to have used was vile," an unnamed source told The Sun.
Nottinghamshire police confirmed a 28-year-old male prison officer had been arrested and since bailed on suspicion of conspiracy to cause actual bodily harm.
The force added: "A further seven members of staff have been suspended from duty while an investigation is carried out."
HMP Nottingham, in the Sherwood area of the city, is a men's Category B prison with a capacity for 1,060 inmates. Nine per cent of the population is Muslim.
An inspection in February found the prison "faced many significant challenges", including high levels of serious violence and use of force by officers.
Inspectors said: "Many prisoners still told us they felt unsafe. High levels of force were used and, while governance of this had improved, we were concerned that some serious allegations made about staff were not being taken seriously enough."
The report found evidence of "collusion" between officers when giving written accounts of incidents, and that legitimate complaints from prisoners about excessive use of force by staff were not adequately investigated.
A survey of inmates also found those from an ethnic background felt more victimised by prison officers than white inmates.
The report added: "In our survey, significantly more Muslim than non-Muslim prisoners said they felt victimised by staff and by other prisoners because of their religion."
The number of assaults in HMP Nottingham has more than doubled between 2010 and 2015, from 139 to 297.