A radical Muslim cleric fighting extradition from the UK could be secretly influencing other inmates, the Prison Officers Association has warned.
Qatada is being held in a specialist unit in Worcestershire
Abu Qatada, who once preached at the Finsbury Park mosque in north London, is being held at a specialist unit at Long Lartin jail in Worcestershire.
Abu Qatada, once described as Osama bin Laden's right-hand man in Europe, is sometimes allowed to read prayers.
But the union said there was no way of knowing exactly what was being said.
The Jordanian national is fighting extradition to his home country where he has been convicted for terror attacks.
Steve Gough, vice-chairman of the Prison Officers Association, said: "The type of people that we're now putting into prison, who have the ability to radicalise and have got a proven track record to radicalise, need a new type of control that we're not ready for and the Government doesn't appear to have put any thought into.
"It is the case that Abu Qatada, and there maybe others, I don't really know, are using prayer in a context that we don't understand.
"The reality is that most of the staff don't know what he's doing."
A Prison Service spokeswoman said inmates at Long Lartin did pray as a group but were only allowed to read prayers on "rare" occasions.
"The small number of detainees in the specialist unit at Long Lartin prison pray as a group three times a day and individually in their cells twice a day," she said.
"The communal prayers are led by the prison's Imam who visits the unit on a daily basis.
"On the rare occasions when an Imam is unable to attend then a prisoner is chosen to read out the prayer of the day."