An investigation has been launched after it emerged that inmates were transferred between two prisons immediately before official inspections were due to start, the Ministry of Justice said today.
Prisoners were moved between Pentonville and Wandsworth prisons in London during inspections in May and June.
Justice Secretary Jack Straw commissioned the investigation after concerns were raised by the chief inspector of prisons, Dame Anne Owers.
He said it was "neither policy nor acceptable practice temporarily to move prisoners during inspections".
The inquiry will look at who suggested and approved the transfers, why, and how many prisoners were involved.
Mr Straw said: "Phil Wheatley, the director general of the National Offender Management Service, has commissioned a formal investigation into information brought to us by Dame Anne Owers, the chief inspector of prisons, concerning the temporary transfer of prisoners in anticipation of HMCIP inspections of two prisons.
"The Prisons and Probation Ombudsman, Stephen Shaw, has also drawn our attention to the transfers.
"The Chief Inspector will make her own judgements in her inspection reports on the prisons, due for release within the next six weeks, but it is neither policy nor acceptable practice temporarily to move prisoners during inspections.
"The terms of reference for the investigation are:
"To investigate the circumstances surrounding the temporary transfer of a number of prisoners between HMPs Pentonville and Wandsworth immediately prior to the HMCIP inspections of each prison.
"In particular, I ask you to examine: who proposed and authorised the transfers; the rationale for the transfers; and the circumstances of the actual transfers themselves and whether those transfers took place in line with policy requirements relating to the well-being of prisoners."
The investigation is expected to be completed within a month.