Great Western Ambulance Service staff in Bristol, Wiltshire, Gloucestershire and parts of Somerset have voted in favour of industrial action.
Unison held the ballot after a row over pay and conditions with their employer the Great Western Ambulance NHS Trust.
The union claimed some workers could face pay cuts and non-peak cover in areas such as Bristol, Gloucester, Bath and Trowbridge could be “slimmed down”.
Unison said it planned to meet the NHS trust later.
Simon Newell, of Unison, said: “Members have not taken any action up until now, but the results clearly demonstrate that our members feel these changes are detrimental to their working conditions.
“With the help of Acas, we hope to reach an amicable resolution with the trust on Friday.
“However, with over 96% of our members saying ‘yes’ to some form of action, our members have left us no doubt what our next steps should be.”
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