Ambulance crews covering south east Wales are refusing to work any overtime for four weeks to highlight staff shortages in the service.
The BBC reported that paramedics say they are worried that patient safety is being compromised because the service is dependant on their willingness to work extra hours.
It is understood hundreds of staff in Cardiff and the Vale of Glamorgan will take part in the unofficial action.
The Welsh Ambulance Services NHS Trust said it was trying to address concerns.
