Unions say the East of England Ambulance Service is failing to crackdown on enforced overtime for its crews.
Staff in North Norfolk recently worked a 20.5 hour day after having to drive a patient to London at the end of their rostered shift.
Unison says it is normal for crews to work two or three hours extra on a single shift.
It is set to ballot its members.
Unison said a survey of 1541 staff found enforced overtime was the top cause of stress for front-line works and the main reason many were considering leaving the ambulance trust.
The union said 319 front-line staff had left their roles over the past 12 months.
At the moment, ambulance crews can be given an emergency call up to the second before the end of their planned shift, regardless of where they are in the trust, or how far they have to travel back to their base station.