Prison staff went on strike in a row over pensions and benefits.
Workshop and administrative workers at The Mount in Bovingdon walked out this morning under national ballot action called by the Public and Commercial Services union.
Union representative Jagtar Singh said: “They want us to pay more, work longer and get less at the end - it doesn’t make sense.
“We are willing to talk to them and negotiate on things but they are unwilling to negotiate.
“This is the most serious dispute we have faced as a union.”