Workers at Great Western Ambulance Service in Bristol, Wiltshire, Gloucestershire and parts of Somerset say they will begin industrial action on Thursday of this week.
Members of the Unison union are planning a work to rule after management demanded they sign new contracts changing their shifts.
Unison activists say that strikes cannot be ruled out.
Bosses replaced ten-hour shifts with 12-hour ones at the beginning of November. Around 800 staff in Bristol and the west of England are affected.