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.
The paramedics said they would work only their contracted hours until 16 January to highlight that the service relied on their goodwill to work overtime.
Some claimed they were working an extra 50 hours a month and it was difficult to take all their annual leave.
One paramedic, who wanted to remain anonymous, said: "You should not have to rely on people doing overtime to cover a service.
"With our staffing levels it's increasingly difficult for people to take their annual leave entitlements without asking colleagues to switch onto their shifts."
A spokesman for the Welsh Ambulance Services NHS Trust said: "We are aware of the unofficial overtime ban and are working hard to address the concerns of staff."
Meanwhile, a queue of 10 ambulances formed outside the University Hospital of Wales in Cardiff on Sunday as the accident and emergency department experienced a backlog of patients.
