Union officials claim patients could be put at risk by changes proposed by the Westcountry Ambulance Service.
The union claims there will be no crew cover when staff take breaks
Unison is to ballot members on plans to close some ambulance stations, and lose 24-hour staffing in others. Part-time staff will also be brought in.
The union claims if shift times are changed it will leave gaps in cover - particularly overnight.
Westcountry Ambulance Service said it will give full details of the plans on Tuesday and address concerns raised.
Ambulance staff say pay is not the contentious issue, although some will receive a substantial pay rise as a result of the changes.
The union says the biggest issue is patient care. It claims the proposals mean more lone working, which could delay a patient going to hospital.
It also says there will be no crew cover when staff take meal breaks because they will no longer be paid for them.