Prison staff across England and Wales have begun protest action after a report warned of a "dangerous lack of control" at HMP Bedford.
Members of the POA trade union were called to demonstrate outside prisons from 7am.
The Chief Inspector of Prisons raised alarm over the potential for a "complete breakdown" in order and discipline at the Bedfordshire prison.
The report warned inmates have effectively taken control at the violent, overcrowded jail.
It also said conditions at the prison were awful with rat and cockroach infestations and high levels of drug abuse.