Prison inspectors have highlighted serious concerns after discovering staff at one jail used a pillowcase as an improvised "hood" to stop an inmate spitting.
Not all staff involved recorded the use of the hood in their reports, and it was unclear if they had got the right authorisation in the first place, said a new report from HM Inspectorate of Prisons.
According to inspectors, the incident was among some issues "that caused serious concern" at HMP Bullingdon in Bicester, Oxfordshire.
Although deemed "improving", the report from an unannounced inspection in July highlighted concerns about the use of a body belt and improvised "hood" to stop the inmate spitting at staff.
"We were particularly concerned about the use of a body belt and improvised hood (a pillowcase) on one prisoner," the report said.
"In addition to our serious concern about the incident itself, we were also disturbed that the report to the use of force committee responsible for reviewing the incident did not disclose the use of either the body belt or the hood."
HMP Bullingdon is a category C training prison which also serves courts in Oxfordshire and Berkshire. Staff at the prison used force 74 times between January and June 2012, compared with 102 times in the same period last year, the report said.
Inspectors said they heard "repeated concerns from a variety of sources about the behaviour of a very small minority of officers" and prisoners had little confidence in the system for making complaints about staff.
Nick Hardwick, chief inspector of prisons, said exceptions to a generally positive picture were relatively few but were "very serious and undermined the work of the prison as a whole", saying they needed to be addressed as a matter of urgency.
Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said: "We would hope this incident was an aberration as the use of force by staff should always be carefully managed and used only in exceptional circumstances. While there are positive aspects to Bullingdon's latest inspection report, there are some issues of serious concern which prison managers need to address."