Additional places have been created at HMP Bullingdon
A jail in Oxfordshire has been told it must take more prisoners, despite being full, to ease general prison overcrowding.
The Ministry of Justice said without using HMP Bullingdon, in Bicester, it would be forced to use police cells.
But a prison officer union leader said the move was unacceptable and inmates would be more susceptible to self-harm.
The Prison Service said an "operational emergency was declared recently" to ensure additional places were created.
The Prison Service said the jail's operational capacity currently stood at 994 inmates.
The figure is the total number of prisoners that an establishment can hold "taking into account control, security and the proper operation of the planned regime", according to the service.
It would not say how many extra inmates would be housed at the prison but said a risk assessment was in place.
HMP Cardiff was also told to take more prisoners to ease general overcrowding.
In a statement, the Prison Service said: "We take our responsibility to hold prisoners safely and securely extremely seriously - no prison is expected to operate at a level of crowding beyond that agreed by area managers and governors.
"An operational emergency was declared recently to ensure additional places identified by two prisons were brought on stream immediately to avoid police cells having to be used unnecessarily to house prisoners.
"Risk assessments were in place in both prisons to ensure the additional prisoners could be accommodated safely."
Glyn Travis, of the Prison Officers Association, said the move would leave inmates feeling "vulnerable".
"Prisoners will be locked up in their cells for longer. That will leave prisoners feeling isolated," he said.
"And the ultimate problem is the fact that prisoners will then self-harm or, in the worse extremes, take a life because there's not sufficient staff or people to cope and manage with the system."
HMP Bullingdon operates as an adult male category C training prison with a category B local function.