Ambulance staff will stage a 12-hour strike at lunchtime today.
Members of Unite in Yorkshire are striking in a dispute over the introduction of emergency care assistants.
About 500 ambulance workers are expected to take part in the industrial action.
Yorkshire Ambulance Services NHS Trust said contingency plans were in place.
The row centres on plans to make savings of £46m over the next five years, including introducing emergency care assistants to work with more highly-trained paramedics.