A prison's population could increase by more than a fifth after an expansion to its estate was formally approved.
HMP Bullingdon, near Bicester, Oxfordshire, currently has about 1,100 inmates but a new building to house another 247 was approved on Thursday.
The expansion will also include new offices, workshops, a multi-faith building and a store.
A planning agent for the Ministry of Justice (MoJ) said the project was of "vital importance".
Chris Hays, who spoke on behalf of the MoJ at a Cherwell District Council planning meeting on Thursday, said the plan was "relatively small but an absolutely essential component of the ministry's expansion".
"Over the next two years the MoJ needs to deliver a critical programme of expansion across the prison estate," he said.
"At present there is a real risk that there will be insufficient accommodation for the number of prisoners expected over the next few years and this proposal is part of the ministry's accelerated house block delivery programme."
The council's planning committee passed the plan unanimously.
A plan for a new prison in Grendon Underwood, about seven miles (11.2km) from HMP Bullingdon in Upper Arncott, was rejected by Buckinghamshire Council in March.