Prison officers are protesting outside HMP Wakefield against working conditions.
Officers at the Love Lane prison joined thousands of colleagues across the country in stopping work due to health and safety concerns.
It comes after talks between the Prison Officers Association (POA) and the government over working conditions broke down.
Andrew Nolan, chair of POA’s Wakefield branch, said: “Over successive years we’ve been haemorrhaging staff at a rate that we can’t retain or recruit.
“Jails are becoming more and more dangerous.
“We can’t carry on like this.
“Locally the issues aren’t as bad as they are in a lot of jails but as you can see nationally, jails are becoming unsafe for staff and for prisoners; the people that we are meant to hold safely.
“We’re holding a day of action today to highlight the plight of the prison service.
“We’ve covered patrols so prisoners will be locked up today.
“Anybody that needs medication will get it, and people will be fed.
“It will be a very restricted regime but safe.”
Mr Nolan, who has worked in the prison service for 30 years, confirmed that a small number of union members had stayed inside the prison during ‘the action’ to “keep the place safe.”
He added: “Because of the situation nationally, jails have become such dangerous places.
“This is a very rare event and officially we’re not allowed to take industrial action.
“But what we’ve done is retreat to the gate under health and safety grounds.
“Until the National Offender Management Services do something, the chances are that more incidents will happen.”
The POA say up to 10,000 members are taking part in the day of protest, which would be “interpreted as a strike”, after talks with the government broke down.
It follows high-profile incidents including two inmates escaping from a London jail and inmates at Bedford prison causing almost £1m worth of damage during a riot.
The Ministry of Justice described the action as unjustified and “unlawful”.
An MoJ spokesman said: “We have well-established contingencies in place to manage prisons and keep the public safe, but we are clear that this constitutes unlawful industrial action, and we will seek remedy in the courts.”