Around 20 prison officers at Birmingham jail are refusing to carry out suicide watch duties, the BBC has learned.
The move is in protest at national overtime payments
The move, not officially voted on but sanctioned by the Prison Officers Association (POA), is part of a national overtime payments protest.
It is thought the action - started on Saturday - could spread to other jails.
Home Office minister Gerry Sutcliffe is due to meet the POA later, although the Home Office said the action was not adversely affecting the prison regime.
In a statement, the POA said officers were not involved in industrial action, but had "chosen to comply with National Union policy".
"Staff are currently being ordered to cover this work which is additional to our current agreed working profile," the POA said.
"This is causing staff a great deal of discomfort and disruption."
The POA insists that "suicide watches are still taking place as normal and are and always will be of the highest priority."
The protest at Birmingham began when some officers refused to monitor inmates at the highest risk of suicide.
The duty, known as "constant watch", involves observing a prisoner in a cell 24 hours a day.
Officers also withdrew from "bedwatch" which involves monitoring inmates in hospital.
Prison governors had to fill in, and one was understood to have worked a 27-hour shift, according to BBC home affairs correspondent Danny Shaw.
He said that although other prison staff had now volunteered, there was concern that the jail would be able to cope for only a few more days.
One prison official said the action was "hugely disruptive" and could be unlawful.
Officers at two other prisons - Liverpool and Blakenhurst, in Worcestershire - are expected to decide later whether to take part in the protest.