Bullingdon Prison near Bicester has more prisoners in its cells than it officially has space for.
Latest figures from the Ministry of Justice reveal there are 964 prisoners - one more than the prison's official capacity of 963.
A ministry spokesman said the prison had more spare cells above its operational capacity, but would not reveal how many spaces were left. Nationally, the number of prison inmates is at an all-time high of 82,972 prisoners.
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