Inmates were moved between Wandsworth and Pentonville prisons
Inmates were moved between two London prisons before each were due to undergo inspections, it has been revealed.
An inquiry into the transfers has begun after concerns were raised by chief inspector of prisons Dame Anne Owers.
Prisoners were transferred between Pentonville and Wandsworth prisons before inspections in May and June.
Justice Secretary Jack Straw said: "It is neither policy nor acceptable practice temporarily to move prisoners during inspections."
'Difficult' prisoners moved
The month-long probe will look into how many prisoners were moved, the reason behind the decision and the official who authorised the move.
It is believed some "difficult" prisoners were moved during the inspections which raised concerns about attempts to deceive inspectors.
Dame Anne said: "I received information about prisoner transfers out of Wandsworth and Pentonville prisons for the duration of their inspections.
"Because of the seriousness of this allegation, and the implications for the prisoners involved, I immediately sent inspectors in to investigate.
"The information that they obtained has been passed on to the Prison Service who have commissioned a formal investigation."
She added the inspection reports of the prisons will contain all the findings.
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 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."
Juliet Lyon, director of the Prison Reform Trust, said: "The integrity of the Prison Service and the independence of our Prisons Inspectorate are respected worldwide, so any action that could lead to prisoners being used as pawns in a game to undermine these institutions must be thoroughly investigated."
Inspections at the country's largest prison, Wandsworth Prison, in south London, took place between 1 and 5 June. It currently holds 1,650 inmates.
Pentonville Prison in north London was inspected between 1 and 11 June and holds 1,168 prisoners.
A 2004 inspection found Wandsworth Prison was "failing to meet basic standards of decency", staff were "actively disrespectful" and prisoners reported "racist comments and behaviour".
A 2006 inspection described accommodation at Pentonville Prison as being like a "shared lavatory", and found prisoners were treated as a "lower order".
Prison inspections look into the condition of cells, the treatment of inmates, the relationship between staff and prisoners and issues such as overcrowding.
The inquiry report is expected to be out by the end of September.